[CMake] Build Fuchsia toolchain as -O3

[mcgrathr]

LGTM
Since we use lld, -Wl,-O2 also helps make the binaries smaller.

[ruiu]

Yes, specifically, lld does string tail merging (instead of regular string merging) when -O2 is given, so it would produce slightly smaller outputs.

[phosek]

It seems like with the current patch, CMake already sets -Wl,-O3 so that should be sufficient.

cfe/trunk/cmake/caches/Fuchsia-stage2.cmake

 # Ideally this would be settable as a per-target option.
 set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
 
 if(APPLE)
   set(LLDB_CODESIGN_IDENTITY "" CACHE STRING "")
 endif()
 
 set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
-set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")
-set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")
+set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
+set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O3 -gline-tables-only -DNDEBUG" CACHE STRING "")
 
 set(LLVM_BUILTIN_TARGETS "x86_64-fuchsia;aarch64-fuchsia" CACHE STRING "")
 foreach(target x86_64;aarch64)
   set(BUILTINS_${target}-fuchsia_CMAKE_SYSROOT ${FUCHSIA_${target}_SYSROOT} CACHE PATH "")
   set(BUILTINS_${target}-fuchsia_CMAKE_SYSTEM_NAME Fuchsia CACHE STRING "")
 endforeach()
 
 if(NOT APPLE)