nss-3.90 breaks w/ earlier x86-64 CPUs (SIGILL)

nss-3.90 unconditionally uses adcxq on x86-64 which breaks on < Broadwell.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1836925. There's an upstream patch to just rip it out for now, but you need a pre-requisite patch too.

Patches:
1. https://github.com/nss-dev/nss/commit/c07c4e073d95a25343cbf56b4a830a71e432869e
2. https://phabricator.services.mozilla.com/D180068?download=true

Hi Sam!

Thanks for the heads-up! It's unfortunately a common problem with the Mozilla codebase and has happened multiple times before.

Someone contributes new code to accelerate functionality using new processor features and then they completely forget to guard this code properly so it isn't used unconditionally.

Also, Mozilla's build system is rather complex which means adding such guards as an external contributor is quite time consuming, been there, done that.

Adrian

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