x86: Allow use of global_data with EFI

On x86 the global_data pointer is provided through a somewhat-bizarre and
x86-specific mechanism: the F segment register is set to a pointer to the
start of global_data, so that accesses can use this build-in register.

When running as an EFI application we don't want to mess with the Global
Descriptor Table (GDT) and there is little advantage (in terms of code size)
to doing so.

Allow global_data to be a simple variable in this case.

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

arch/x86/include/asm/global_data.h

@@ -76,6 +76,12 @@ struct arch_global_data {
 #include <asm-generic/global_data.h>
 
 #ifndef __ASSEMBLY__
+# ifdef CONFIG_EFI_APP
+
+#define gd global_data_ptr
+
+#define DECLARE_GLOBAL_DATA_PTR   extern struct global_data *global_data_ptr
+# else
 static inline __attribute__((no_instrument_function)) gd_t *get_fs_gd_ptr(void)
 {
 	gd_t *gd_ptr;
@@ -88,6 +94,7 @@ static inline __attribute__((no_instrument_function)) gd_t *get_fs_gd_ptr(void)
 #define gd	get_fs_gd_ptr()
 
 #define DECLARE_GLOBAL_DATA_PTR
+# endif
 
 #endif