firefox: Fix AArch64 build

 * The 'arm.patch' patch doesn't apply anymore.
 * The 'build-arm-libopus.patch' patch isn't required anymore.
 * See the mozilla phabricator link for the added patch.

Additionally, we are now *always* undconditionally applying all patches
to all architectures. That is, unless they have undesirable
side-effects, but those might not be fit for inclusion.

By applying all patches all the time, they'll be removed or replaced
when they stop applying.