LoongArch: Implement C[LT]Z_DEFINED_VALUE_AT_ZERO

LoongArch: Implement C[LT]Z_DEFINED_VALUE_AT_ZERO

The LoongArch has defined ctz and clz on the backend, but if we want GCC
do CTZ transformation optimization in forwprop2 pass, GCC need to know
the value of c[lt]z at zero, which may be beneficial for some test cases
(like spec2017 deepsjeng_r).

After implementing the macro, we test dynamic instruction count on
deepsjeng_r:
- before 1688423249186
- after  1660311215745 (1.66% reduction)

gcc/ChangeLog:

	* config/loongarch/loongarch.h (CLZ_DEFINED_VALUE_AT_ZERO):
	Implement.
	(CTZ_DEFINED_VALUE_AT_ZERO): Same.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr90838.c: add clz/ctz test support on LoongArch.