GCC: fix ResetCanaryAndReturn unit test

GCC does not support the attribute optnone, so ResetCanaryAndReturn
was optimized and failing to assert as expected.

For GCC it should use optimize(0) attribute.

Bug: 819294
Change-Id: I3ca2bccf1d4b0893f4b91f91c306f05454699ee4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4992982


Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1218980}