target-mips: add unions to access DSP elements

target-mips: add unions to access DSP elements

Instead of playing with bit shifting, add two unions (one for 32-bit
values, one for 64-bit ones) to access all the DSP elements with the
correct type.

This make the code easier to read and less error prone, and allow GCC
to vectorize the code in some cases.

Reviewed-by: Eric Johnson <ericj@mips.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>