[tsan] Use internal_sigfillset to replace REAL(sigfillset).

Summary:
Android doesn't intercept sigfillset, so REAL(sigfillset) is null.
And we can use internal_sigfillset() for all cases.

Reviewers: kcc, eugenis, kubabrecka, dvyukov

Subscribers: llvm-commits, tberghammer, danalbert

Differential Revision: http://reviews.llvm.org/D15296

llvm-svn: 257862