[tests] Use __SSE2__ (rather than __i686__...) for SSE2 ASAN test

Use the __SSE2__ to determine whether SSE2 is enabled in the ASAN tests
rather than relying on either of the __i686__ and __x86_64__. The former
is only set with explicit -march=i686, and therefore misses most of
the x86 CPUs that support SSE2. __SSE2__ is in turn defined if
the current settings (-march, -msse2) indicate that SSE2 is supported
which should be more reliable.

Differential Revision: https://reviews.llvm.org/D26763

llvm-svn: 287245