x86, efi: Fix processor-specific memcpy() build error

x86, efi: Fix processor-specific memcpy() build error

Building for Athlon/Duron/K7 results in the following build error,

arch/x86/boot/compressed/eboot.o: In function `__constant_memcpy3d':
eboot.c:(.text+0x385): undefined reference to `_mmx_memcpy'
arch/x86/boot/compressed/eboot.o: In function `efi_main':
eboot.c:(.text+0x1a22): undefined reference to `_mmx_memcpy'

because the boot stub code doesn't link with the kernel proper, and
therefore doesn't have access to the 3DNow version of memcpy. So,
follow the example of misc.c and #undef memcpy so that we use the
version provided by misc.c.

See https://bugzilla.kernel.org/show_bug.cgi?id=50391

Reported-by: Al Viro <[email protected]>
Reported-by: Ryan Underwood <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: [email protected]
Signed-off-by: Matt Fleming <[email protected]>