tcg/mips: Support unaligned access for user-only

tcg/mips: Support unaligned access for user-only

This is kinda sorta the opposite of the other tcg hosts, where
we get (normal) alignment checks for free with host SIGBUS and
need to add code to support unaligned accesses.

Fortunately, the ISA contains pairs of instructions that are
used to implement unaligned memory accesses.  Use them.

Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>