rseq/selftests: Implement MIPS support

Implement support for both MIPS32 & MIPS64 in the rseq selftests, in
order to sanity check the recently enabled rseq syscall.

The tests all pass on a MIPS Boston development board running either a
MIPS32r2 interAptiv CPU & a MIPS64r6 I6500 CPU, both of which were
configured with 2 cores each of which have 2 hardware threads (VP(E)s) -
ie. 4 CPUs.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Reviewed-by: James Hogan <jhogan@kernel.org>
Patchwork: https://patchwork.linux-mips.org/patch/19524/
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org