When porting to MacOS (in https://crrev.com/c/1282960), we forgot to
update signal masking to unmask SIGBUS instead of SIGSEGV.

R=mseaborn@chromium.org, ahaas@chromium.org

Bug: chromium:906565
Change-Id: I486a204f33139f1da4a074a07878921692b713e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3015565
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75680}