AIX: .csect support for XCOFFAsmParser

Hello I'm compiling V8 on AIX and ran into an issue in the backend not supporting `.csect` directive. I'm making this issue to request adding support for it.

```text
Fatal error: error in backend: XCOFFAsmParser directive not yet supported!
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0. Program arguments: /home/llvm/clang+llvm-20.1.7-powerpc64-ibm-aix-7.2/bin/clang++ -MD -MF obj/v8_heap_base/push_registers_asm.o.d -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D__LIBC_NO_CPP_MATH_OVERLOADS__ -D_GNU_SOURCE -D_ALL_SOURCE=1 -D_LIBCPP_HARDENING_MODE=_LIBCPP_HARDENING_MODE_EXTENSIVE -D_LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCXXABI_DISABLE_VISIBILITY_ANNOTATIONS -D_LIBCPP_INSTRUMENTED_WITH_ASAN=0 -DCR_LIBCXX_REVISION=2c359c239b138a20a03f798e47889448ef131c22 -DNDEBUG -DNVALGRIND -DDYNAMIC_ANNOTATIONS_ENABLED=0 -DV8_TYPED_ARRAY_MAX_SIZE_IN_HEAP=64 -DV8_INTL_SUPPORT -DV8_USE_EXTERNAL_STARTUP_DATA -DV8_ATOMIC_OBJECT_FIELD_WRITES -DV8_ENABLE_LAZY_SOURCE_POSITIONS -DV8_WIN64_UNWINDING_INFO -DV8_ENABLE_REGEXP_INTERPRETER_THREADED_DISPATCH -DV8_ENABLE_FUZZTEST -DV8_ENABLE_SPARKPLUG -DV8_ENABLE_TURBOFAN -DV8_ENABLE_WEBASSEMBLY -DV8_ENABLE_CONTINUATION_PRESERVED_EMBEDDER_DATA -DV8_ALLOCATION_FOLDING -DV8_ALLOCATION_SITE_TRACKING -DV8_ADVANCED_BIGINT_ALGORITHMS -DV8_USE_ZLIB -DV8_USE_LIBM_TRIG_FUNCTIONS -DV8_ENABLE_MAGLEV_GRAPH_PRINTER -DV8_ENABLE_EXTENSIBLE_RO_SNAPSHOT -DV8_ENABLE_BLACK_ALLOCATED_PAGES -DV8_ENABLE_LEAPTIERING -DV8_WASM_RANDOM_FUZZERS -DV8_ARRAY_BUFFER_INTERNAL_FIELD_COUNT=0 -DV8_ARRAY_BUFFER_VIEW_INTERNAL_FIELD_COUNT=0 -DV8_PROMISE_INTERNAL_FIELD_COUNT=0 -DV8_DEPRECATION_WARNINGS -DV8_IMMINENT_DEPRECATION_WARNINGS -DCPPGC_ENABLE_LARGER_CAGE -DCPPGC_SLIM_WRITE_BARRIER -DV8_TARGET_ARCH_PPC64 -DV8_RUNTIME_CALL_STATS -DABSL_ALLOCATOR_NOTHROW=1 -I../.. -Igen -I../../buildtools/third_party/libc++ -I../../include -Igen/include -I../../third_party/abseil-cpp -Wall -Wextra -Wimplicit-fallthrough -Wextra-semi -Wunreachable-code-aggressive -Wthread-safety -Wgnu -Wno-gnu-anonymous-struct -Wno-gnu-conditional-omitted-operand -Wno-gnu-include-next -Wno-gnu-label-as-value -Wno-gnu-redeclared-enum -Wno-gnu-statement-expression -Wno-gnu-zero-variadic-macro-arguments -Wno-zero-length-array -Wno-missing-field-initializers -Wno-unused-parameter -Wno-psabi -Wloop-analysis -Wno-unneeded-internal-declaration -Wno-cast-function-type -Wno-thread-safety-reference-return -Wshadow -fno-delete-null-pointer-checks -fno-strict-overflow -fno-ident -fno-strict-aliasing -funwind-tables -fcolor-diagnostics -fmerge-all-constants -fno-sized-deallocation -fcrash-diagnostics-dir=../../tools/clang/crashreports -ffp-contract=off -fcomplete-member-pointers -Wall -Wno-unused-parameter -pthread -Wmissing-field-initializers -Wno-uninitialized -mfprnd -maix64 -fdata-sections -ffunction-sections -O3 --target=powerpc64-ibm-aix -Wno-builtin-macro-redefined -D__DATE__= -D__TIME__= -D__TIMESTAMP__= -no-canonical-prefixes -fomit-frame-pointer -g0 -Wheader-hygiene -Wstring-conversion -Wtautological-overlap-compare -ffp-contract=off -mfprnd -Wunreachable-code -Wno-shadow -Wctad-maybe-unsupported -Wno-invalid-offsetof -Wmissing-field-initializers -O3 -fno-math-errno -Wexit-time-destructors -Wno-invalid-offsetof -Wenum-compare-conditional -Wno-nullability-completeness -std=c++20 -Wno-trigraphs -gsimple-template-names -fno-rtti -fno-exceptions -Wno-narrowing -Wno-non-virtual-dtor -fno-exceptions -fno-rtti -nostdinc++ -isystem../../third_party/libc++/src/include -isystem../../third_party/libc++abi/src/include -c ../../src/heap/base/asm/ppc/push_registers_asm.cc -o obj/v8_heap_base/push_registers_asm.o
1. parser at end of file
2. Code generation
clang++: error: clang frontend command failed with exit code 70 (use -v to see invocation)
clang version 20.1.7 (https://github.com/llvm/llvm-project.git 6146a88f60492b520a36f8f8f3231e15f3cc6082)
Target: powerpc64-ibm-aix
Thread model: posix
InstalledDir: /home/llvm/clang+llvm-20.1.7-powerpc64-ibm-aix-7.2/bin
clang++: note: diagnostic msg:
********************
```

The code that is getting compiled was the following:

`-c ../../src/heap/base/asm/ppc/push_registers_asm.cc -o obj/v8_heap_base/push_registers_asm.o`

```c
#if defined(_AIX)
".globl .PushAllRegistersAndIterateStack, hidden \n"
".csect .text[PR] \n"
".PushAllRegistersAndIterateStack: \n"

...
```

https://chromium.googlesource.com/v8/v8/+/11781fed53b2aa18a087ec7f5984ef5445146e63/src/heap/base/asm/ppc/push_registers_asm.cc#22

I was able to workaround this by passing `-fno-integrated-as` but I would like to not have to pass this flag everywhere and just use the integrated as.

Start with the XCOFFAsmParser entry point and reproduce the failure using the reported clang++ command on AIX. Inspect how assembler directives are dispatched, using V8's src/heap/base/asm/ppc/push_registers_asm.cc and its .csect usage as the case to support. Done means the integrated assembler accepts this input without -fno-integrated-as and has regression coverage.

Written by the indexing model from the issue text.