LoongArch: Add ifunc support for strrchr{aligned, lsx, lasx}

LoongArch: Add ifunc support for strrchr{aligned, lsx, lasx}

According to glibc strrchr microbenchmark test results, this implementation
could reduce the runtime time as following:

Name                Percent of rutime reduced
strrchr-lasx        10%-50%
strrchr-lsx         0%-50%
strrchr-aligned     5%-50%

Generic strrchr is implemented by function strlen + memrchr, the lasx version
will compare with generic strrchr implemented by strlen-lasx + memrchr-lasx,
the lsx version will compare with generic strrchr implemented by strlen-lsx +
memrchr-lsx, the aligned version will compare with generic strrchr implemented
by strlen-aligned + memrchr-generic.