x86/xen: remove 32-bit awareness from startup_xen

startup_xen is still 32-bit aware, even if no longer needed.

Replace the register macros by the 64-bit register names for making
it more readable.
Signed-off-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20211028081221.2475-5-jgross@suse.comReviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

arch/x86/xen/xen-head.S

@@ -45,13 +45,13 @@ SYM_CODE_START(startup_xen)
 
 	/* Clear .bss */
 	xor %eax,%eax
-	mov $__bss_start, %_ASM_DI
-	mov $__bss_stop, %_ASM_CX
-	sub %_ASM_DI, %_ASM_CX
-	shr $__ASM_SEL(2, 3), %_ASM_CX
-	rep __ASM_SIZE(stos)
+	mov $__bss_start, %rdi
+	mov $__bss_stop, %rcx
+	sub %rdi, %rcx
+	shr $3, %rcx
+	rep stosq
 
-	mov %_ASM_SI, xen_start_info
+	mov %rsi, xen_start_info
 	mov initial_stack(%rip), %rsp
 
 	/* Set up %gs.