target/mips: Introduce ase_msa_available() helper

Instead of accessing CP0_Config3 directly and checking
the 'MSA Present' bit, introduce an explicit helper,
making the code easier to read.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201208003702.4088927-2-f4bug@amsat.org>