[runtimes] Probe for -nostdlib++ and -nostdinc++ with the C compiler (#108357)

While these flags semantically are relevant only for C++, we do add them
to CMAKE_REQUIRED_FLAGS if they are detected. All flags in that variable
are used both when testing compilation of C and C++ (and for detecting
libraries, which uses the C compiler driver).

Therefore, to be sure we safely can add the flags to
CMAKE_REQUIRED_FLAGS, test for the option with the C language.

This should fix compilation with GCC; newer versions of GCC do support
the -nostdlib++ option, but it's only supported by the C++ compiler
driver, not the C driver. (However, many builds of GCC also do accept
the option with the C driver, if GCC was compiled with Ada support
enabled, see [1]. That's why this issue isn't noticed in all
configurations with GCC.)

Clang does support these options in both C and C++ driver modes.

This should fix #90332.

[1]
llvm/llvm-project#90332 (comment)

NOKEYCHECK=True
GitOrigin-RevId: 75d0281bc81f0040c24d15bdf9c5cc46e9237224

Author: mstorsjo

cmake/config-ix.cmake

@@ -34,9 +34,13 @@ endif()
 # required during compilation (which has the -nostdlib++ or -nodefaultlibs). libc is
 # required for the link to go through. We remove sanitizers from the
 # configuration checks to avoid spurious link errors.
+#
+# Adding flags to CMAKE_REQUIRED_FLAGS will include the flags both when testing
+# compilation of C and C++. Therefore test to make sure that the flags are
+# supported by the C compiler driver, before deciding to include them.
 
-llvm_check_compiler_linker_flag(CXX "-nostdlib++" CXX_SUPPORTS_NOSTDLIBXX_FLAG)
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+llvm_check_compiler_linker_flag(C "-nostdlib++" C_SUPPORTS_NOSTDLIBXX_FLAG)
+if (C_SUPPORTS_NOSTDLIBXX_FLAG)
   set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -nostdlib++")
 else()
   llvm_check_compiler_linker_flag(C "-nodefaultlibs" C_SUPPORTS_NODEFAULTLIBS_FLAG)
@@ -47,7 +51,7 @@ endif()
 
 # Only link against compiler-rt manually if we use -nodefaultlibs, since
 # otherwise the compiler will do the right thing on its own.
-if (NOT CXX_SUPPORTS_NOSTDLIBXX_FLAG AND C_SUPPORTS_NODEFAULTLIBS_FLAG)
+if (NOT C_SUPPORTS_NOSTDLIBXX_FLAG AND C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (LIBUNWIND_HAS_C_LIB)
     list(APPEND CMAKE_REQUIRED_LIBRARIES c)
   endif ()
@@ -82,7 +86,7 @@ if (NOT CXX_SUPPORTS_NOSTDLIBXX_FLAG AND C_SUPPORTS_NODEFAULTLIBS_FLAG)
   endif()
 endif()
 
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
+if (C_SUPPORTS_NOSTDLIBXX_FLAG OR C_SUPPORTS_NODEFAULTLIBS_FLAG)
   if (CMAKE_C_FLAGS MATCHES -fsanitize OR CMAKE_CXX_FLAGS MATCHES -fsanitize)
     set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -fno-sanitize=all")
   endif ()

src/CMakeLists.txt

@@ -66,7 +66,7 @@ set(LIBUNWIND_SOURCES
     ${LIBUNWIND_ASM_SOURCES})
 
 # Generate library list.
-if (CXX_SUPPORTS_NOSTDLIBXX_FLAG)
+if (C_SUPPORTS_NOSTDLIBXX_FLAG)
   add_link_flags_if_supported(-nostdlib++)
 else()
   if (LIBUNWIND_USE_COMPILER_RT)