libc: Use the initial-exec TLS model

This permits more efficient accesses of thread-local variables, which
are heavily used at least by jemalloc and locale-aware code. Note that
on amd64 and i386, jemalloc's thread-local variables already have their
TLS model overridden by defining JEMALLOC_TLS_MODEL.

For now the change is applied only to tested platforms, but should in
principle be enabled everywhere.

PR: 255840
Suggested by: jrtc27
Reviewed by: kib
Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9c97062b620137a1f7cad4c6b3fb030a396b3266)