libc/string/bcopy.c: Use intptr_t as the copy type

While most 64-bit architectures have an assembly implementation of this
file RISC-V does not. As we now copy 8 bytes instead of 4 it should speed
up RISC-V. Using intptr_t instead of int also allows using this file for
CHERI pure-capability code since trying to copy pointers using integer
loads/stores will invalidate pointers.

Reviewed By: kib
Obtained from: CheriBSD (partially)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D29535