target-mips: Provide function to test if a CPU supports an ISA

Provide a new cpu_supports_isa function which allows callers to
determine whether a CPU supports one of the ISA_ flags, by testing
whether the associated struct mips_def_t sets the ISA flags in its
insn_flags field.

An example use of this is to allow boards which generate bootloader code
to determine the properties of the CPU that will be used, for example
whether the CPU is 64 bit or which architecture revision it implements.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Yongbok Kim <yongbok.kim@imgtec.com>