xen arm/arm64: minor improvement in smp_send_call_function_mask()

xen arm/arm64: minor improvement in smp_send_call_function_mask()

Currently, smp_send_call_function_mask() function implemented
by xen arm/arm64 will use IPI to call function on current CPU.

This means that current smp_send_call_function_mask() will do
the following on current CPU:
Trigger SGI for current CPU => Xen takes interrupt on current CPU
=> IPI interrupt handler will call smp_call_function_interrupt()

This patch improves the above by straight away calling
smp_call_function_interrupt() for current CPU. This is very
similar to smp_send_call_function_mask() implemented by Xen x86.

Signed-off-by: Anup Patel <anup.patel@linaro.org>
Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@linaro.org>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>