MIPS 64-bit ISA is introduced with MIPS3.

Introduce the CPU_MIPS64 definition aliased to the MIPS3 ISA,
and the cpu_type_is_64bit() method to check if a CPU supports
this ISA (thus is 64-bit).

Suggested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210104221154.3127610-5-f4bug@amsat.org>