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

include/longlong.h

@@ -593,6 +593,18 @@ extern UDItype __umulsidi3 (USItype, USItype);
 #define UMUL_TIME 14
 #endif
 
+#ifdef __loongarch__
+# if W_TYPE_SIZE == 32
+#  define count_leading_zeros(count, x)  ((count) = __builtin_clz (x))
+#  define count_trailing_zeros(count, x) ((count) = __builtin_ctz (x))
+#  define COUNT_LEADING_ZEROS_0 32
+# elif W_TYPE_SIZE == 64
+#  define count_leading_zeros(count, x)  ((count) = __builtin_clzll (x))
+#  define count_trailing_zeros(count, x) ((count) = __builtin_ctzll (x))
+#  define COUNT_LEADING_ZEROS_0 64
+# endif
+#endif
+
 #if defined (__M32R__) && W_TYPE_SIZE == 32
 #define add_ssaaaa(sh, sl, ah, al, bh, bl) \
   /* The cmp clears the condition bit.  */ \