arm64: entry: Use SPSel to switch to overflow stack

When detecting a stack overflow on exception entry from EL1, use SPSel
to switch to the overflow stack without corrupting any GPRs. Not only is
this simpler than the previous logic, but it also opens the door to
more complicated overflow checks (for example, based on per-task stack
sizes or stacks which are not aligned to a specific page order) as well
as the possibility of returning from a stack fault if we were able to
resolve it.

Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>