libc: fix _FORTIFY_SOURCE build on aarch64

As with their amd64 counterparts, we need to undef these string funcs
that _FORTIFY_SOURCE will have defined macros for to avoid breaking the
build.

Fixes: bea89d038ac ("lib/libc/aarch64/string: add strlcat SIMD [...]")
Fixes: 3dc5429158c ("lib/libc/aarch64/string: add strncat SIMD [...]")