[compiler-rt] Don't change g_tls_size after initialization.

Summary:
g_tls_size is not supposed to be changed after initialization. It's not
atomic, not guarded by a lock, nor thread_local. But it's read by
multiple threads.

The reason why it's mutated is mips and powerpc64 specific. We can
implement the same funcitonality without mutating g_tls_size.

I'm not sure how to write a test for this. Please advice. Thanks!

Reviewers: eugenis, kcc

Subscribers: kubamracek, dberris, llvm-commits

Differential Revision: https://reviews.llvm.org/D29236

llvm-svn: 293586