Libzstd: SHSTK not supported in cmake

## Problem description

SHSTK is an x64 security feature that is part of Intel CET, and support for it was added in PR #2992. **The problem is**, this PR only applies to the *make* build system, and not the *cmake* one, so when built with cmake, `libzstd.so` is still not marked as SHSTK-compatible, and this causes SHSTK not to be supported on applications that use libzstd.

## How to reproduce

On systems that use cmake to build the library:

```console
user@fedora:~$ readelf -n /usr/lib64/libzstd.so | grep SHSTK
... (no results) ...
```

## Proposed fix

The crux of the issue is that when cmake compiles `lib/decompress/huf_decompress_amd64.S`, it doesn't pass any flags to the compiler. The object therefore gets compiled without `-fcf-protection`, which causes `cet.h` not to emit a `.note.gnu.property` section in the object, which in turn causes the final `libzstd.so` not to have that section either.

This can be fixed by simply using the C compiler flags when building assembly files, to ensure that `-fcf-protection` gets passed on them:

```diff
diff --git a/build/cmake/CMakeModules/AddZstdCompilationFlags.cmake b/build/cmake/CMakeModules/AddZstdCompilationFlags.cmake
index 3e7bcce5..cddb7ceb 100644
--- a/build/cmake/CMakeModules/AddZstdCompilationFlags.cmake
+++ b/build/cmake/CMakeModules/AddZstdCompilationFlags.cmake
@@ -113,6 +113,9 @@ macro(ADD_ZSTD_COMPILATION_FLAGS _C _CXX _LD)
endif ()
endif ()

+ # Use the C flags as ASM flags
+ set(CMAKE_ASM_FLAGS "${CMAKE_C_FLAGS}")
+
# Remove duplicates compilation flags
foreach (flag_var CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO
```

Confirmed that it fixes the issue:

```console
user@fedora:~$ readelf -n libzstd.so.1.5.7 | grep SHSTK
Properties: x86 feature: IBT, SHSTK
```