opcodes: make use of __builtin_popcount when available

opcodes: make use of __builtin_popcount when available

This commit provides a small performance improvement when starting up
CGEN based disassemblers by making use of __builtin_popcount.

The #if check used in this commit was copied from bfd/elf32-arm.c
where __builtin_popcount is also used.

I ran into this code while investigating some GDB tests that would
occasionally timeout.  One of the reason these tests were having
problems is that the m16c and m32c disassemblers take so long to
initialise themselves.  Speeding up count_decodable_bits helps, but is
not a total solution.  Still, this felt like an easy win which added
minimal extra complexity, so I figure its worth doing.

opcodes/ChangeLog:

	* cgen-dis.c (count_decodable_bits): Use __builtin_popcount when
	available.