libc: Avoid defining memcpy() as a macro under FORTIFY_SOURCE

[markj]

Summary

Otherwise it is impossible to make certain memcpy() usages work, e.g.,
memcpy(dst, &(struct foo){1, 2}, sizeof(*dst)) fails to compile
because a macro parameter can't contain a comma. This arises when
compiling graphics/mesa-libs with FORTIFY_SOURCE enabled.