linux-user: implement HWCAP bits on MIPS

Add support for the two currently defined HWCAP bits on MIPS - R6 and
MSA.

Buglink: https://bugs.launchpad.net/qemu/+bug/1754372


Signed-off-by: James Cowgill <james.cowgill@mips.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20180315151348.6451-1-james.cowgill@mips.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>