target/mips/translate: Expose check_mips_64() to 32-bit mode

To allow compiling 64-bit specific translation code more
generically (and removing #ifdef'ry), allow compiling
check_mips_64() on 32-bit targets.
If ever called on 32-bit, we obviously emit a reserved
instruction exception.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20201215225757.764263-3-f4bug@amsat.org>