libc: Disable ASAN for certain string functions

[arichardson]

Summary

They deliberately read out-of-bounds values to avoid byte-by-byte
loads and check multiple bytes at once. While this will work on x86,
it is flagged as an out-of-bounds read with ASAN, so we have to
disable instrumentation here. This also causes bounds errors for CHERI,
so in CheriBSD we use implementations that avoid OOB reads.

[arichardson]

remove debug

[arichardson]

Add memchr