arm64: kernel: Add arch-specific SDEI entry code and CPU masking

The Software Delegated Exception Interface (SDEI) is an ARM standard
for registering callbacks from the platform firmware into the OS.
This is typically used to implement RAS notifications.

Such notifications enter the kernel at the registered entry-point
with the register values of the interrupted CPU context. Because this
is not a CPU exception, it cannot reuse the existing entry code.
(crucially we don't immediately know which exception level we interrupted),

Add entry.S asm to set us up for calling into C code. If the event
interrupted code that had interrutps masked, we always return to that
location. Otherwise we pretend this was an IRQ, and use SDEI's
complete_and_resume call to return to vbar_el1 + offset.

This allows the kernel to deliver signals to user space processes. For
KVM this triggers the world switch, a quick spin round vcpu_run, then
back into the guest, unless there are pending signals.

Add sdei_mask_local_cpu() calls to the smp_send_stop() code, this covers
the panic() code-path, which doesn't invoke cpuhotplug notifiers, and
only calls the panic notifiers if there is no kdump kernel registered.

Because we can interrupt entry-from/exit-to EL0, we can't trust the value
in sp_el0 even if we interrupted the kernel, in this case the code in
entry.S will save/restore sp_el0 and use the value in __entry_task.

Signed-off-by: James Morse <james.morse@arm.com>