arm: initialize gd for AArch64

Commit adc421e4 "arm: move gd handling outside of C code" removed
the call to arch_setup_gd() on ARM and replaced it with assembly code
in crt0.S. However, AArch64 uses a different startup file, and the same
change was not made to it. This leaves gd uninitialized on AArch64, which
typically leads to hangs or crashes. This change fixes that.

Fixes: adc421e4 ("arm: move gd handling outside of C code")
Signed-off-by: Stephen Warren <swarren@nvidia.com>

arch/arm/lib/crt0_64.S

@@ -78,6 +78,8 @@ ENTRY(_main)
 	mov	x0, sp
 	bl	board_init_f_alloc_reserve
 	mov	sp, x0
+	/* set up gd here, outside any C code */
+	mov	x18, x0
 	bl	board_init_f_init_reserve
 
 	mov	x0, #0