Big endian isn't an officially supported platform upstream, and as far as I'm aware, none of our big endian ports are actually using Skia anyway, so we should just disable it from being built.

Just voicing some discussion that was had in the Toronto office on Bugzilla... The reason I ended up whitelisting x86/arm instead of disabling BE is that really, the platforms that upstream tests against regularly are x86 and arm and not other configurations. Whilst other platforms are "supported", there's no real guarantee that they won't be broken. As the usage of Skia in Firefox for non-x86/arm platforms is pretty much non-existent, I think it's wise to only enable building it on x86/arm.