lib/x86_64/setjmp: Use 32-bit zero idiom for shorter encoding

Switch from "xorq %rax, %rax" to "xorl %eax, %eax". In 64-bit mode
zeroing EAX implicitly clears RAX and the 32-bit form encodes are one
byte smaller while keeping identical semantics.

Signed-off-by: George Hu <integral@archlinux.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/lib/x86_64/setjmp.S

@@ -36,7 +36,7 @@ GRUB_MOD_LICENSE "GPLv3+"
  */
 FUNCTION(grub_setjmp)
 	pop	%rsi		/* Return address, and adjust the stack */
-	xorq	%rax, %rax
+	xorl	%eax, %eax
 	movq	%rbx, 0(%rdi)	/* RBX */
 	movq	%rsp, 8(%rdi)   /* RSP */
 	push	%rsi