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 implicitly know which exception level we interrupted),

Add the entry point to entry.S to set us up for calling into C code. If
the event interrupted code that had interrupts 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.

Because we can interrupt entry-from/exit-to another EL, we can't trust the
value in sp_el0 or x29, 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.

When we have VMAP stacks we can interrupt the stack-overflow test, which
stirs x0 into sp, meaning we have to have our own VMAP stacks. For now
these are allocated when we probe the interface. Future patches will add
refcounting hooks to allow the arch code to allocate them lazily.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>

CC: bogus bogus bogus

Changes since v4:
 * Moved asm code into kernel/entry.S to make the kaiser interaction smaller
 * Added preempt.h for in_nmi() declaration
 * Made use of __uaccess_enable_hw_pan()
 * Forced GFP_KERNEL means we can lazily allocate stacks, fixed comments saying
   we can't do this, patches to follow...

Changes since v3:
 * Added const to clobbered_registers,
 * Removed extern from C function declarations.
 * Header file names changed

Changes since v2:
 * Added PAN-setting as we didn't take an exception to get here.
 * Added VMAP stack allocation and switching.
 * overwrite x29 on entry from not-the-kernel.
 * Added a pe-mask to crash_smp_send_stop()s 'no secondaries' path (oops).
 * Added ELR-hasn't-changed test. Any exception in the handler will panic KVM
   as its switched VBAR_EL1, but go silently undetected otherwise. Generate a
   warning.

fix comment - stacks will become lazy

PAN helper