platform independent way to choose libvpx optimization code

[punto-]

Godot version:
2.x, 3.x

Issue description:
We need a way to choose which platform code is used for libvpx in a way that is platform independent (ie not depending on the value of env['platform']) for platforms that are not known to the module. At least have it default to the software implementation code (if that exists), at the moment I think it's just not compiling any platform specific code (which is fine), but later the linker fails because the code doesn't exist anywhere. This one is a bit trickier because it depends on the CPU type and the compiler, but maybe expose some flags to let the platform override all the logic we have right now in libvpx/SCsub?