x86: Set up toolchain flags for running as EFI application

Adjust the toolchain flags to build U-Boot as a relocatable shared library,
as required by EFI.

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