Fix TLS support check for Darwin 32-bit simulator targets.

Also instead of checking architecture explicitly, use recently added
"simulator" environment in the triple.

rdar://problem/35083787

Reviewers: arphaman, bob.wilson

Reviewed By: arphaman

Subscribers: gparker42, cfe-commits

Differential Revision: https://reviews.llvm.org/D41750

llvm-svn: 321890