Add support for AArch64 to spidermonkey

[andrew]

Summary

This adds support for arm64 to the spidermonkey 170 and 24 ports. Both of these are failing with:

#error Target architecture was not detected as supported by Double-Conversion.

This is based on https://hg.mozilla.org/integration/mozilla-inbound/rev/6a46f53ad944

The spidermonkey 17 and 185 ports are also failing, however in different ways.

Test Plan

I've built lang/spidermonkey170 and lang/spidermonkey24 on ThunderX under Poudriere to verify they build.