ignore the piuparts failures for gcc-7-cross, gcc-8-cross

[Andreas Beckmann <anbe@debian.org>]

Hi,

please

ignore-piuparts gcc-7-cross/23
ignore-piuparts gcc-8-cross/17

The failures are in cruft packages. There is a manual decruft requested:
#899045, but the cruft packages shouldn't block migration anyway.
piuparts does not know about cruft, it just takes everything from the
Packages files.

Andreas