target/mips: Fix MSA instructions ST.<B|H|W|D> on big endian host

Fix the case when the host is a big endian machine, and change
the approach toward ST.<B|H|W|D> instruction helpers.

Signed-off-by: Mateja Marjanovic <mateja.marjanovic@rt-rk.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Message-Id: <1554212605-16457-3-git-send-email-mateja.marjanovic@rt-rk.com>