x86_64: Fix build with --disable-multiarch (BZ 30721)

With multiarch disabled, the default memmove implementation provides
the fortify routines for memcpy, mempcpy, and memmove.  However, it
does not provide the internal hidden definitions used when building
with fortify enabled.  The memset has a similar issue.

Checked on x86_64-linux-gnu building with different options:
default and --disable-multi-arch plus default, --disable-default-pie,
--enable-fortify-source={2,3}, and --enable-fortify-source={2,3}
with --disable-default-pie.
Tested-by: Andreas K. Huettel <dilfridge@gentoo.org>
Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>