This would allow SP to be a nonsense value, which would confuse the CPU
profiler when the signal handler read the SP before it was restored.

Bug: v8:8355
Change-Id: If108c8cf00467904fe0f4f26fddc2e3122afcd93
Reviewed-on: https://chromium-review.googlesource.com/c/1298032Reviewed-by: Martyn Capewell <martyn.capewell@arm.com>
Reviewed-by: Alexei Filippov <alph@chromium.org>
Commit-Queue: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56967}