[compiler_rt] buildgo.sh warnings

I see a bunch of warnings when building compiler_rt on OSX, e.g.,

[4574/6448] Building CXX object projects/compiler-rt/lib/tsan/CMakeFiles/RTTsan_dynamic.ios.dir/rtl/tsan_ignoreset.cc.o
clang: warning: argument unused during compilation: ‘-msse3’ [-Wunused-command-line-argument]

The problem is that “-msse3” is always added if $DEBUG is undefined/empty. However, I don’t see where/how DEBUG is getting set – even though I’m running a debug build of llvm + compiler_rt – so this branch is always taken.