arm64: perf: don't rely on layout of pt_regs when grabbing sp or pc

The current perf_regs code relies on sp and pc sitting just off the end
of the pt_regs->regs array. This is ugly and fragile, so this patch
checks for these register explicitly and returns the appropriate field.

Acked-by: Jean Pihet <jean.pihet@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>