x86: Support building the EFI stub

Add support for building a 32/64-bit EFI stub for x86. This involves
building the startup and relocation code for either i386 or x86_64.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
diff --git a/arch/x86/lib/Makefile b/arch/x86/lib/Makefile
index 43489fd..09c236b 100644
--- a/arch/x86/lib/Makefile
+++ b/arch/x86/lib/Makefile
@@ -34,7 +34,7 @@
 obj-$(CONFIG_CMD_ZBOOT)	+= zimage.o
 obj-$(CONFIG_HAVE_FSP) += fsp/
 
-extra-$(CONFIG_USE_PRIVATE_LIBGCC) := lib.a
+extra-$(CONFIG_USE_PRIVATE_LIBGCC) += lib.a
 
 NORMAL_LIBGCC = $(shell $(CC) $(PLATFORM_CPPFLAGS) -print-libgcc-file-name)
 OBJCOPYFLAGS := --prefix-symbols=__normal_