cmake: fix CXX flag dependency in AddZstdCompilationFlags when C++ is not enabled

[Tyagiquamar]

Summary of changes

Fixes #4800.

Since #4357, the CMake build enables C++ only when tests are enabled (ZSTD_BUILD_TESTS=ON), but AddZstdCompilationFlags.cmake still invoked CHECK_CXX_COMPILER_FLAG whenever CMAKE_CXX_COMPILER was defined in the environment or toolchain (such as standard MinGW/cross-compilation toolchains).

When C++ is not enabled as a CMake project language:

1. CHECK_CXX_COMPILER_FLAG fails with check_compiler_flag: CXX: needs to be enabled before use.
2. The compiler flags guard checked CMAKE_CXX_COMPILER_ID (which is empty when C++ is not loaded), causing default C-only builds to lose standard warnings (-Wall, -Wextra, etc.) and assembly support.
3. ZSTD_HAS_NOEXECSTACK unconditionally required CXX_FLAG_WA_NOEXECSTACK to be set.

This change:

• Updates EnableCompilerFlag to check C++ compiler flags only when CMAKE_CXX_COMPILER_LOADED is true.
• Updates the GNU/Clang compiler ID check in ADD_ZSTD_COMPILATION_FLAGS to evaluate CMAKE_C_COMPILER_ID and CMAKE_CXX_COMPILER_ID.
• Passes the actual variables ${_C}, ${_CXX}, ${_LD} to EnableCompilerFlag.
• Allows ZSTD_HAS_NOEXECSTACK to succeed in C-only mode without requiring C++ CXX_FLAG_WA_NOEXECSTACK.

Validation

• Clean build in Docker container (cmake -S build/cmake -B /tmp/zstd-build -G Ninja -DCMAKE_BUILD_TYPE=Release && cmake --build /tmp/zstd-build).
• Verified all C warning flags (-Wall, -Wextra, -Wshadow, etc.), libzstd.a, libzstd.so, and executable binaries build and link with exit code 0.