restructure nlr.h for undefined archtectures

[jerryneedell]

This PR is by no means critical - there may be better ways to do this
Background
I was attempting to build CP on a Raspberry Pi with Ubuntu 19.04 installed - just to see if it worked...
When I tried to build mpy-cross - it failed with several instances of this error

make: Entering directory '/home/ubuntu/circuitpython/mpy-cross'
Use make V=1, make V=2 or set BUILD_VERBOSE similarly in your environment to increase build verbosity.
GEN build/genhdr/moduledefs.h
In file included from ../py/mpstate.h:34,
                 from ../py/runtime.h:29,
                 from ../py/gc.c:32:
../py/nlr.h:63: error: "MICROPY_NLR_SETJMP" redefined [-Werror]
     #define MICROPY_NLR_SETJMP (1)
 
../py/nlr.h:39: note: this is the location of the previous definition
 #define MICROPY_NLR_SETJMP (0)

The Ubuntu distribution on RPi uses the architecture "aarch64" which is not recognized by nlr.h. This should not necessarily be a problem, but as written nlr.h attempts to redefine MICROPY_NLR_SETJMP if the architecture is not recognized.
This PR just restructures the tests so it is only defined once.

With this change, the build of mpy-cross succeeds.
This change has no impact on the BSP builds other than providing a working mpy-cross.

After all taht, it is not clear there is a real need to be able to build under Ubuntu on an RPi since the build tools work fine under Raspian Buster.

The main reason for this change is to fix a potential error when unrecognized architectures are used. nlr.h should not try to redefine MICROPY_NLR_SETJMP.

I think this is a "harmless" change to avoid that.

Note: I also did a little "clean up" of the indentation to make the section more readable.

[tannewt]

Looks good to me! Thanks @jerryneedell