Add optimized versions of musl libc string and memory comparison functions.
Add strcoll as well so that library.js does not need to depend on musl libc code.
Add missing memcmp symbol declaration from previous commit.
Revert strcmp.c to musl v0.9.13 so that the versions in this pull request match the ones that waywardmonkeys has pushed in earlier.
Fix test_strcmp_uni and test_dlfcn_stacks after previous commit.
Merge pull request #2103 from juj/opt_cmp

Add optimized versions of musl libc string and memory comparison functions.