[ubsan-minimal] Fix the x86_64h config check

Checking if config.target_arch is x86_64h doesn't work (the 'h' suffix
is dropped here, and I didn't account for that). Instead, check to see
if '-arch x86_64h' is in the cflags.

Tested on a pre-Haswell bot.

rdar://problem/34378605

llvm-svn: 313053