Working on reconciling out-of-tree patches to compiler-rt for building for iOS.

Summary:
This is one of many changes needed for compiler-rt to get it building on iOS.

This change does the following:
- Don't include crt_externs on iOS (it isn't available)
- Support ARM thread state objects

Note: this change does not enable building for iOS, as there are more changes to come.

Reviewers: glider, kubabrecka, bogner, samsonov

Reviewed By: samsonov

Subscribers: samsonov, aemerson, llvm-commits

Differential Revision: http://reviews.llvm.org/D10510

llvm-svn: 240467