hw_x86_64: Finally restore RAX client register value

Set the stack to the mode transition buffer and pop the temporary copy
of the RAX client value into the register.

repos/base-hw/src/core/spec/x86_64/mode_transition.s

@@ -119,6 +119,10 @@
 	mov CR3_OFFSET(%rax), %rax
 	mov %rax, %cr3
 
+	/* Set stack back to mt buffer and restore client RAX */
+	mov $_mt_buffer, %rsp
+	popq %rax
+
 	1: jmp 1b
 
 	/* end of the mode transition code */