Test ARCH=um

[nickdesaulniers]

I think @kees sent patches for this recently.
https://lore.kernel.org/lkml/202203210821.9E5FDCA863@keescook/

[nathanchance]

tuxsuite needs to add support for um as a target_arch value to match tuxmake:

https://gitlab.com/Linaro/tuxsuite/-/issues/157

[nathanchance]

#348