[sanitizer] Remove -fno-function-sections from SANITIZER_COMMON_CFLAGS

[cryptoad]

Summary

-fno-function-sections was added as a default Sanitizer common cflag with
https://reviews.llvm.org/rL200683, the reasoning behind was that things would
break if linked with --gc-sections.

This appears to not be necessary anymore, as tests pass without, including
function-sections-are-bad.cc. There is a large benefit to having
function-sections when dealing with static libraries in terms of size and
dependencies that go away with --gc-sections.

[eugenis]

LGTM

CMakeLists.txt

 if(NOT COMPILER_RT_DEBUG)
   append_list_if(COMPILER_RT_HAS_FOMIT_FRAME_POINTER_FLAG -fomit-frame-pointer SANITIZER_COMMON_CFLAGS)
 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_FUNCTION_SECTIONS_FLAG -fno-function-sections SANITIZER_COMMON_CFLAGS)
 append_list_if(COMPILER_RT_HAS_FNO_LTO_FLAG -fno-lto SANITIZER_COMMON_CFLAGS)
 
 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