the kuser_helpers are disabled by default on grsec. we target
arm6+ so make arm atomics be inline assembly always.

ref #2522

add the missing hunks from gregor's firefox patchset

Based heavily on GregorR's musl-pkgsrc-patches firefox patch,
but slightly differs at places to suit Alpine needs.

Submitted upstream:
https://bugzilla.mozilla.org/show_bug.cgi?id=878791#c7

Fixes issues with addressbar not working in firefox-20 and probably
also the firefox-19 issues:

https://bugzilla.mozilla.org/show_bug.cgi?id=843564
https://bugzilla.mozilla.org/show_bug.cgi?id=843513