target/mips: Use 8-byte memory ops for msa load/store

target/mips: Use 8-byte memory ops for msa load/store

Rather than use 4-16 separate operations, use 2 operations
plus some byte reordering as necessary.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>