arm: fix build after c/s 25477:e12e0b038219

Only x86 currently has a struct vcpu field arch.gdbsx_vcpu_event. But
as the whole function domain_pause_for_debugger() is pointless to be
compiled when there's no arch support, simply introduce another HAS_*
macro, enabled only on x86.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -51,6 +51,7 @@ CFLAGS-$(perfc)         += -DPERF_COUNTE
 CFLAGS-$(perfc_arrays)  += -DPERF_ARRAYS
 CFLAGS-$(lock_profile)  += -DLOCK_PROFILE
 CFLAGS-$(HAS_ACPI)      += -DHAS_ACPI
+CFLAGS-$(HAS_GDBSX)     += -DHAS_GDBSX
 CFLAGS-$(HAS_PASSTHROUGH) += -DHAS_PASSTHROUGH
 CFLAGS-$(frame_pointer) += -fno-omit-frame-pointer -DCONFIG_FRAME_POINTER
 
--- a/xen/arch/x86/Rules.mk
+++ b/xen/arch/x86/Rules.mk
@@ -9,6 +9,7 @@ HAS_PASSTHROUGH := y
 HAS_NS16550 := y
 HAS_EHCI := y
 HAS_KEXEC := y
+HAS_GDBSX := y
 xenoprof := y
 
 #
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -612,6 +612,7 @@ void vcpu_end_shutdown_deferral(struct v
         vcpu_check_shutdown(v);
 }
 
+#ifdef HAS_GDBSX
 void domain_pause_for_debugger(void)
 {
     struct domain *d = current->domain;
@@ -628,6 +629,7 @@ void domain_pause_for_debugger(void)
     if (current->arch.gdbsx_vcpu_event == 0)
         send_global_virq(VIRQ_DEBUGGER);
 }
+#endif
 
 /* Complete domain destroy after RCU readers are not holding old references. */
 static void complete_domain_destroy(struct rcu_head *head)