[NFC][PowerPC] add test cases for milicode (#175559)
In this PR, we do the following:
1. Simplify the test case for the millicode function `___memmove`.
2. Add test cases for the millicode functions `___memcpy` ,
`____memset`, `____memmove` which are supported in the patch
https://reviews.llvm.org/D143997.
3. Add pre-commit test cases for the functions `___strstr`,
`___memccpy`, `___strcmp`
