target/mips: Use CP0_Config3 to set MIPS_HFLAG_MSA

MSA presence is expressed by the MSAP bit of CP0_Config3.
We don't need to check anything else.

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: <20201208003702.4088927-4-f4bug@amsat.org>