Fix gc-test.cc to work under higher -O levels

The gc-test.cc tries underflows of a variable up to -32 bytes, but on i386, the left redzone is not 32 bytes, it’s only 16 bytes and therefore the access to var[-32] is completely off. The reason why this test didn’t fail before is that we’ve been lucky and there was another variable before the var array, which was also instrumented. This fix uses “-32” for 64-bit systems and “-16” for 32-bit.

Reviewed at http://reviews.llvm.org/D7809

llvm-svn: 230172