mips: Don't set __NO_TLS to disable some uses of TLS.

NO_TLS was originally added to disable use of _Thread in the locale
code in libc in 82dd5016bd749d1d9e1531bd1703aebeecceab34. At the time
libc did not support TLS on MIPS (I believe), but TLS support was
added to libc (at least _set_tp.c) for MIPS about a month after
NO_TLS was added, but __NO_TLS was still left around.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D28713