[CMake] Enable -qfuncsect when building with IBM XL

[hubert.reinterpretcast]

Summary

The IBM XL compiler uses -qfuncsect for -ffunction-sections.

The comment about sanitizers and -f[no-]function-sections is corrected also, as it is pertinent to this patch.

The sanitizer-related use of -fno-function-sections is associated with powerpc64le, a target for which there is an IBM XL compiler, so that use is updated in this patch to apply -qnofuncsect in case a build using the XL compiler is viable on that platform.

This patch has been verified with the XL compiler on AIX only.

[daltenty]

LGTM

compiler-rt/CMakeLists.txt

 endif()
 append_list_if(COMPILER_RT_HAS_FNO_LTO_FLAG -fno-lto SANITIZER_COMMON_CFLAGS)
 
 # The following is a workaround for powerpc64le. This is the only architecture
 # that requires -fno-function-sections to work properly. If lacking, the ASan
 # Linux test function-sections-are-bad.cpp fails with the following error:
 # 'undefined symbol: __sanitizer_unaligned_load32'.
 if(DEFINED TARGET_powerpc64le_CFLAGS)
+  if(CMAKE_CXX_COMPILER_ID MATCHES "XL")
+    append("-qnofuncsect" TARGET_powerpc64le_CFLAGS)
+  else()
     append_list_if(COMPILER_RT_HAS_FNO_FUNCTION_SECTIONS_FLAG -fno-function-sections TARGET_powerpc64le_CFLAGS)
   endif()
+endif()
 
 # The following is a workaround for s390x.  This avoids creation of "partial
 # inline" function fragments when building the asan libraries with certain
 # GCC versions.  The presence of those fragments, in particular for the
 # interceptors, changes backtraces seen in asan error cases, which causes
 # testsuite failures.
 if("${COMPILER_RT_DEFAULT_TARGET_ARCH}" MATCHES "s390x")
   append_list_if(COMPILER_RT_HAS_FNO_PARTIAL_INLINING_FLAG -fno-partial-inlining SANITIZER_COMMON_CFLAGS)

llvm/cmake/modules/HandleLLVMOptions.cmake

 # FIXME: With MSVS, consider compiling with /Gy and linking with /OPT:REF?
 # But MinSizeRel seems to add that automatically, so maybe disable these
 # flags instead if LLVM_NO_DEAD_STRIP is set.
 if(NOT CYGWIN AND NOT WIN32)
   if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin" AND
      NOT uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
     check_c_compiler_flag("-Werror -fno-function-sections" C_SUPPORTS_FNO_FUNCTION_SECTIONS)
     if (C_SUPPORTS_FNO_FUNCTION_SECTIONS)
-      # Don't add -ffunction-section if it can be disabled with -fno-function-sections.
+      # Don't add -ffunction-sections if it can't be disabled with -fno-function-sections.
       # Doing so will break sanitizers.
       add_flag_if_supported("-ffunction-sections" FFUNCTION_SECTIONS)
+    elseif (CMAKE_CXX_COMPILER_ID MATCHES "XL")
+      append("-qfuncsect" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     endif()
     add_flag_if_supported("-fdata-sections" FDATA_SECTIONS)
   endif()
 endif()
 
 if(MSVC)
   # Remove flags here, for exceptions and RTTI.
   # Each target property or source property should be responsible to control