LoongArch: implement count_{leading,trailing}_zeros

LoongArch: implement count_{leading,trailing}_zeros

LoongArch always support clz and ctz instructions, so we can always use
__builtin_{clz,ctz} for count_{leading,trailing}_zeros.  This improves
the code of libgcc, and also benefits Glibc once we merge longlong.h
there.

Bootstrapped and regtested on loongarch64-linux-gnu.

include/ChangeLog:

	* longlong.h [__loongarch__] (count_leading_zeros): Define.
	[__loongarch__] (count_trailing_zeros): Likewise.
	[__loongarch__] (COUNT_LEADING_ZEROS_0): Likewise.