Builds fail on musl systems

Commit 1e145f2 accidentally closed #2167, but the issue is not fixed. I still get the following error when building Folly on a Musl system (Gentoo Musl in my case):

```
FAILED: CMakeFiles/folly_base.dir/folly/io/RecordIO.cpp.o
/usr/bin/x86_64-pc-linux-musl-g++ -DBOOST_ATOMIC_DYN_LINK -DBOOST_ATOMIC_NO_LIB -DBOOST_CONTEXT_DYN_LINK -DBOOST_CONTEXT_NO_LIB -DBOOST_FILESYSTEM_DYN_LINK -DBOOST_FILESYSTEM_NO_LIB -DBOOST_PROGRAM_OPTIONS_DYN_LINK -DBOOST_PROGRAM_OPTIONS_NO_LIB -DBOOST_REGEX_DYN_LINK -DBOOST_REGEX_NO_LIB -DBOOST_SYSTEM_DYN_LINK -DBOOST_SYSTEM_NO_LIB -DBOOST_THREAD_DYN_LINK -DBOOST_THREAD_NO_LIB -DFMT_SHARED -DFOLLY_XLOG_STRIP_PREFIXES=\"/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00:/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00_build\" -DGFLAGS_IS_A_DLL=0 -D_GNU_SOURCE -D_REENTRANT -I/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00 -I/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00_build -I/usr/include/libiberty -O2 -pipe -std=gnu++17 -fPIC -fcoroutines -g -finput-charset=UTF-8 -fsigned-char -Wall -Wno-deprecated -Wno-deprecated-declarations -Wno-sign-compare -Wno-unused -Wuninitialized -Wunused-label -Wunused-result -Wshadow-compatible-local -Wno-noexcept-type -faligned-new -fopenmp -MD -MT CMakeFiles/folly_base.dir/folly/io/RecordIO.cpp.o -MF CMakeFiles/folly_base.dir/folly/io/RecordIO.cpp.o.d -o CMakeFiles/folly_base.dir/folly/io/RecordIO.cpp.o -c /var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/io/RecordIO.cpp
In file included from /var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/io/RecordIO.h:36,
from /var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/io/RecordIO.cpp:17:
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:100:5: error: 'off64_t' does not name a type; did you mean 'off_t'?
100 | off64_t pageSize = 0;
| ^~~~~~~
| off_t
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:144:32: error: 'off64_t' has not been declared
144 | MemoryMapping(AnonymousType, off64_t length, Options options = Options());
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:148:7: error: 'off64_t' has not been declared
148 | off64_t offset = 0,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:149:7: error: 'off64_t' has not been declared
149 | off64_t length = -1,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:154:7: error: 'off64_t' has not been declared
154 | off64_t offset = 0,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:155:7: error: 'off64_t' has not been declared
155 | off64_t length = -1,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:160:7: error: 'off64_t' has not been declared
160 | off64_t offset = 0,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:161:7: error: 'off64_t' has not been declared
161 | off64_t length = -1,
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:250:13: error: 'off64_t' has not been declared
250 | void init(off64_t offset, off64_t length);
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:250:29: error: 'off64_t' has not been declared
250 | void init(off64_t offset, off64_t length);
| ^~~~~~~
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:254:3: error: 'off64_t' does not name a type; did you mean 'off_t'?
254 | off64_t mapLength_ = 0;
| ^~~~~~~
| off_t
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h: In member function 'folly::MemoryMapping::Options& folly::MemoryMapping::Options::setPageSize(off_t)':
/var/tmp/portage/dev-cpp/folly-2024.11.04.00/work/folly-2024.11.04.00/folly/system/MemoryMapping.h:73:7: error: 'pageSize' was not declared in this scope; did you mean 'setPageSize'?
73 | pageSize = v;
| ^~~~~~~~
| setPageSize
```

Relevant commit from Abseil suggesting `off_t` should be used instead: https://github.com/abseil/abseil-cpp/commit/4500c2fada4e952037c59bd65e8be1ba0b29f21e