[CMake] Reland "Make simple source used for CMake checks a C file"

[phosek]

Summary

The source being compiled is plain C, but using .cc extension forces it
to be compiled as C++ which requires a working C++ compiler including
C++ library which may not be the case when we're building compiler-rt
together with libcxx as part of runtimes build.

CMakeLists.txt

if (SANITIZER_CXX_ABI STREQUAL "default")
   if (HAVE_LIBCXXABI AND COMPILER_RT_DEFAULT_TARGET_ONLY)
     set(SANITIZER_CXX_ABI_LIBNAME "libcxxabi")
     set(SANITIZER_CXX_ABI_INTREE 1)
   elseif (APPLE)
     set(SANITIZER_CXX_ABI_LIBNAME "libcxxabi")
     set(SANITIZER_CXX_ABI_SYSTEM 1)
   else()
     set(SANITIZER_CXX_ABI_LIBNAME "libstdc++")
+    set(SANITIZER_CXX_ABI_SYSTEM 1)
   endif()
 else()
   set(SANITIZER_CXX_ABI_LIBNAME "${SANITIZER_CXX_ABI}")
 endif()
 
 if (SANITIZER_CXX_ABI_LIBNAME STREQUAL "libcxxabi")
   if (SANITIZER_CXX_ABI_INTREE)
     if (NOT LIBCXXABI_ENABLE_STATIC_UNWINDER AND (TARGET unwind_shared OR HAVE_LIBUNWIND))

cmake/Modules/CompilerRTUtils.cmake

elseif(TEST_COMPILE_ONLY)
       try_compile_only(CAN_TARGET_${arch} ${TARGET_${arch}_CFLAGS})
     else()
       set(FLAG_NO_EXCEPTIONS "")
       if(COMPILER_RT_HAS_FNO_EXCEPTIONS_FLAG)
         set(FLAG_NO_EXCEPTIONS " -fno-exceptions ")
       endif()
       set(SAVED_CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS})
       set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${argstring}")
+      if(SANITIZER_CXX_ABI_SYSTEM)
+        set(FLAG_LINK_LIBRARIES ${SANITIZER_CXX_ABI_LIBRARY})
+      endif()
       try_compile(CAN_TARGET_${arch} ${CMAKE_BINARY_DIR} ${SIMPLE_SOURCE}
                   COMPILE_DEFINITIONS "${TARGET_${arch}_CFLAGS} ${FLAG_NO_EXCEPTIONS}"
+                  LINK_LIBRARIES ${FLAG_LINK_LIBRARIES}
                   OUTPUT_VARIABLE TARGET_${arch}_OUTPUT)
       set(CMAKE_EXE_LINKER_FLAGS ${SAVED_CMAKE_EXE_LINKER_FLAGS})
     endif()
   endif()
   if(${CAN_TARGET_${arch}})
     list(APPEND COMPILER_RT_SUPPORTED_ARCH ${arch})
   elseif("${COMPILER_RT_DEFAULT_TARGET_ARCH}" STREQUAL "${arch}" AND
          COMPILER_RT_HAS_EXPLICIT_DEFAULT_TARGET_TRIPLE)

cmake/config-ix.cmake

 
 # List of all architectures we can target.
 set(COMPILER_RT_SUPPORTED_ARCH)
 
 # Try to compile a very simple source file to ensure we can target the given
 # platform. We use the results of these tests to build only the various target
 # runtime libraries supported by our current compilers cross-compiling
 # abilities.
-set(SIMPLE_SOURCE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/simple.cc)
+set(SIMPLE_SOURCE ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/simple.c)
 file(WRITE ${SIMPLE_SOURCE} "#include <stdlib.h>\n#include <stdio.h>\nint main() { printf(\"hello, world\"); }\n")
 
 # Detect whether the current target platform is 32-bit or 64-bit, and setup
 # the correct commandline flags needed to attempt to target 32-bit and 64-bit.
 if (NOT CMAKE_SIZEOF_VOID_P EQUAL 4 AND
     NOT CMAKE_SIZEOF_VOID_P EQUAL 8)
   message(FATAL_ERROR "Please use architecture with 4 or 8 byte pointers.")
 endif()