[sanitizer] Re-enable -fno-function-sections for powerpc64le

[cryptoad]

Summary

For a reason that hasn't been investigated for lack of powerpc knowledge and
hardware, -fno-function-sections is required for the Sanitizers to work
properly on powerpc64le. Without, the function-sections-are-bad test fails on
that architecture (and that architecture only).

This patch re-enables the flag in the powerpc64le cflags.

I have to admit I am not entirely sure if my way is the proper way to do this,
so if anyone has a better way, I'll be happy to oblige.

[eugenis]

This looks OK.
Please mention the test that is failing and the error message either in the comment or in the commit message.

[cryptoad]

Updating the comment to include the failing test and error message.

CMakeLists.txt

 endif()
 append_list_if(COMPILER_RT_HAS_FUNWIND_TABLES_FLAG -funwind-tables SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FNO_STACK_PROTECTOR_FLAG -fno-stack-protector SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FNO_SANITIZE_SAFE_STACK_FLAG -fno-sanitize=safe-stack SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FVISIBILITY_HIDDEN_FLAG -fvisibility=hidden SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FVISIBILITY_INLINES_HIDDEN_FLAG -fvisibility-inlines-hidden SANITIZER_COMMON_CFLAGS)
 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.cc fails with the following error:
+# 'undefined symbol: __sanitizer_unaligned_load32'.
+if(DEFINED TARGET_powerpc64le_CFLAGS)
+  append_list_if(COMPILER_RT_HAS_FNO_FUNCTION_SECTIONS_FLAG -fno-function-sections TARGET_powerpc64le_CFLAGS)
+endif()
+
 if(MSVC)
   # Replace the /M[DT][d] flags with /MT, and strip any definitions of _DEBUG,
   # which cause definition mismatches at link time.
   # FIXME: In fact, sanitizers should support both /MT and /MD, see PR20214.
   if(COMPILER_RT_HAS_MT_FLAG)
     foreach(flag_var
       CMAKE_C_FLAGS CMAKE_C_FLAGS_DEBUG CMAKE_C_FLAGS_RELEASE
       CMAKE_C_FLAGS_MINSIZEREL CMAKE_C_FLAGS_RELWITHDEBINFO