[sanitizer] make __tls_get_addr interceptor (more) AS-safe. No good test so far, will keep trying.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@200734 91177308-0d34-0410-b5e6-96231b3b80d8
Windows does not have the ".const" directive and the function type must be declared differently.
Only Macs use .subsections_via_symbols
-fPIC switch does not apply on Windows.
This file doesn't need to be executable.
Enable building compiler-rt for a specific LLVM triple with GCC.