Making 'file' an implicit native input in core-updates

[mhw]

Hi,

I recently updated libffi, libatomic_ops, and libgc on core-updates.
Unfortunately, all three of them fail on MIPS, for the same reason that
sudo and gawk currently need workarounds: all of these packages were
bootstrapped with a version of 'libtool' that requires /usr/bin/file on
MIPS.

Given that this problem is becoming increasingly common, I think we
should add 'file' as an implicit native-input for the gnu-build-system
in core-updates, and also add a new phase to %standard-phases that
substitutes /usr/bin/file in the usual places with a path that works in
the build environment.

What do you think?

      Mark