Hi Guix,

the first patch is to declare that Julia is unsupported on ARM and MIPS,
as previously discussed on IRC.

The second patch fixes the binary substitutes for Julia on the two
supported architectures by passing the MARCH flag with generic CPUs
("pentium4" is the lowest supported CPU for the i686 architecture).
This is needed as Julia's system image is otherwise targetted to the CPU
of the build host.

~~ Ricardo