[sanitizers] [SystemZ] Add __tls_get_offset interceptor.

s390 is special again - instead of __tls_get_addr, it has __tls_get_offset
with special calling conventions: the result is TP relative, and
the argument is GOT-relative.  Since we need to get address of the caller's
GOT, which is in %r12, we have to use assembly like glibc does.

Aside of __tls_get_offset, glibc also implements a slightly saner
__tls_get_addr_internal, which takes a pointer as argument, but still
returns a TP-relative offset.  It is used for dlsym() called on TLS
symbols, so we have to intercept it was well.  Our __tls_get_offset
is also implemented by delegating to it.

Differential Revision: http://reviews.llvm.org/D19778

llvm-svn: 273041