Basic native ARM emitter

Resolves issue #800.
It closely resembles the thumb2 emitter, due to the similiarities and is feature-wise on par.
A basic fibonacci snippet
````
def fib(n):
    if n == 0:
        return 0
    elif n == 1:
        return 1
    else:
        return fib(n - 1) + fib(n - 2)

print(fib(30))
````
executes sucessfully, as does tests/micropython/viper.py.
The command line works as well. I haven't implemented a better use of registers > R7, as it was unclear for me what to change.
Is 'ARM' a fair name to use for this new emitter, that won't clash with another emitter later on?  I mean, are there any other ARM instruction sets to support?