arm: remove outdated comment concerning -ffixed-x18

Clang 9 supports -ffixed-x18.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/include/asm/global_data.h b/arch/arm/include/asm/global_data.h
index 7c0905d..2aafc6d 100644
--- a/arch/arm/include/asm/global_data.h
+++ b/arch/arm/include/asm/global_data.h
@@ -96,10 +96,6 @@
 	gd_t *gd_ptr;
 
 #ifdef CONFIG_ARM64
-	/*
-	 * Make will already error that reserving x18 is not supported at the
-	 * time of writing, clang: error: unknown argument: '-ffixed-x18'
-	 */
 	__asm__ volatile("mov %0, x18\n" : "=r" (gd_ptr));
 #else
 	__asm__ volatile("mov %0, r9\n" : "=r" (gd_ptr));