Loongarch: Add ifunc support for strchr{aligned, lsx, lasx} and strchrnul{aligned, lsx, lasx}

These implementations improve the time to run strchr{nul}
microbenchmark in glibc as below:
strchr-lasx       reduces the runtime about 50%-83%
strchr-lsx        reduces the runtime about 30%-67%
strchr-aligned    reduces the runtime about 10%-20%
strchrnul-lasx    reduces the runtime about 50%-83%
strchrnul-lsx     reduces the runtime about 36%-65%
strchrnul-aligned reduces the runtime about 6%-10%