gnu-efi compatibility patch: use memset and memcpy from syslinux

Filtering out the memset and memcpy from syslinux (done with the gnu-efi
compatibility patch taken from Ubuntu) causes some modules (e.g.
vesamenu.c32) to fail with "Undef symbol FAIL: memset".

Instead pass the -zmuldefs option to the gnu linker.  This will result
in the linker using the memset/memcpy provided by syslinux and ignore
the one included in gnu-efi.