micropython: Invert fallback detection logic

Rather than specifying architectures that do not have explicit support
in micropython invert the logic and set MICROPY_GCREGS_SETJMP=1 if the
architecture does not have explicit support. MIPS is listed as being
supported but this support consists of automatically defining
MICROPY_GCREGS_SETJMP 1 based on __mips__ being defined.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>