TBA - Support i386 target

We should add support for the x86 target. As far as I can see, this requires a separate build.

We may want a separate .mozconfig for this. Maybe we can keep the shared config lines into .mozconfig-android, and put the different targets and objdirs into .mozconfig-android-x86 and .mozconfig-android-arm.

mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-arm-linux-androideabi                                                                                                                                                    
ac_add_options --target=arm-linux-androideabi

mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/obj-x86-linux-androideabi                                                                                                                                                    
ac_add_options --target=i386-linux-android

When running mach configure for i386, we'll need: rustup target add i686-linux-android and we'll need to install yasm (apt install yasm).