[libc][NFC] Allow memcpy to be inlined

This allows shipping individual functions without also having to provide
`memcpy` at the expense of bigger functions.
Next is to use this `inlined_memcpy` in:
 - loader/linux/x86_64/start.cpp
 - src/string/memmove.cpp
 - src/string/mempcpy.cpp
 - src/string/strcpy.cpp
 - src/string/strdup.cpp
 - src/string/strndup.cpp

Differential Revision: https://reviews.llvm.org/D113097