MIPS: Opt into HAVE_COPY_THREAD_TLS

MIPS: Opt into HAVE_COPY_THREAD_TLS

This the mips version of commit 3f627a3c3befb ("x86: opt into
HAVE_COPY_THREAD_TLS, for both 32-bit and 64-bit").

Simply use the tls system call argument instead of extracting the tls
argument by magic from the pt_regs structure.

See commit 4d047cf174a7f ("clone: support passing tls argument via C
rather than pt_regs magic") for more background.

Signed-off-by: James Cowgill <James.Cowgill@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15855/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>