Call real pthread_attr_getstack instead of the interceptor

Summary:
Call real pthread_attr_getstack instead of the interceptor
when we do intercept pthread_attr_getstack.

Reviewers: samsonov, eugenis

Reviewed By: samsonov

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2237

llvm-svn: 195441