x86: Tidy up availability of string functions

For now, just enable the fast-but-large string functions in 32-boot
U-Boot proper only. Avoid using them in SPL. We cannot use then in 64-bit
builds since we only have 32-bit assembly.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Simon Glass <sjg@chromium.org>