tsan: revert: Update buildgo.sh to pass -isysroot on Darwin.

This commit breaks actual Go runtime build on gomote builders (10.12) with:

xcode-select: error: tool 'xcodebuild' requires Xcode,
but active developer directory '/Library/Developer/CommandLineTools'
is a command line tools instance

Without this part build works fine.
The original commit does not include any explanation as to why
it is needed.

llvm-svn: 327700