efi_loader: allow compiling with clang

On ARM systems gd is stored in register r9 or x18. When compiling with
clang gd is defined as a macro calling function gd_ptr(). So we can not
make assignments to gd.

In the UEFI sub-system we need to save gd when leaving to UEFI binaries and
have to restore gd when reentering U-Boot.

Define a new function set_gd() for setting gd and use it in the UEFI
sub-system.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Tested-by: Tom Rini <trini@konsulko.com>
diff --git a/lib/efi_loader/efi_boottime.c b/lib/efi_loader/efi_boottime.c
index db34938..1591ad8 100644
--- a/lib/efi_loader/efi_boottime.c
+++ b/lib/efi_loader/efi_boottime.c
@@ -49,7 +49,7 @@
  * restriction so we need to manually swap its and our view of that register on
  * EFI callback entry/exit.
  */
-static volatile void *efi_gd, *app_gd;
+static volatile gd_t *efi_gd, *app_gd;
 #endif
 
 /* 1 if inside U-Boot code, 0 if inside EFI payload code */
@@ -89,7 +89,7 @@
 #ifdef CONFIG_ARM
 	assert(efi_gd);
 	app_gd = gd;
-	gd = efi_gd;
+	set_gd(efi_gd);
 #endif
 	return ret;
 }
@@ -99,7 +99,7 @@
 {
 	int ret = --entry_count == 0;
 #ifdef CONFIG_ARM
-	gd = app_gd;
+	set_gd(app_gd);
 #endif
 	return ret;
 }
@@ -123,7 +123,7 @@
 	/* Only restore if we're already in EFI context */
 	if (!efi_gd)
 		return;
-	gd = efi_gd;
+	set_gd(efi_gd);
 #endif
 }
 
@@ -2920,7 +2920,7 @@
 		 * otherwise __efi_entry_check() will put the wrong value into
 		 * app_gd.
 		 */
-		gd = app_gd;
+		set_gd(app_gd);
 #endif
 		/*
 		 * To get ready to call EFI_EXIT below we have to execute the