mips _libc_get_static_tls_base: Narrow scope of #ifdef.

Reviewed by: kib, emaste, jrtc27
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33348

(cherry picked from commit 9952b82b398968a243909ee3854b1968c3cc8c12)