x86/xen: Make hypercall_page noinstr

vmlinux.o: warning: objtool: xen_set_debugreg()+0x3: call to hypercall_page() leaves .noinstr.text section
vmlinux.o: warning: objtool: xen_get_debugreg()+0x3: call to hypercall_page() leaves .noinstr.text section
vmlinux.o: warning: objtool: xen_irq_enable()+0x24: call to hypercall_page() leaves .noinstr.text section

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20210624095148.810950584@infradead.org