The existing cpp-14 tests also learn about the two new spread runners, on
which they are cross-compilation cases.

Matches the marker the binutils and cpp-15 slices carry, and makes it
clear the entry is not the directory it looks like.

The ld-linux-*.so.* glob matches nothing on ppc64el and s390x, whose
loaders are ld64.so.2 and ld64.so.1. Use the same lookup canonical#1090 gave the
binutils tests, so all four cpp-14 suites agree.

Nothing was failing: the value is only consumed on the native-architecture
path, which never runs on those two runners.

Same bug and same fix as the cpp-14 slices this branch builds on: the
/usr/share/doc entry of a gcc-14-<triplet> package is a symlink to
gcc-14-base for the native build and to gcc-14-<triplet>-base for the
cross builds, so pulling gcc-14-base unconditionally left it dangling
everywhere but on the native architecture.

Depends on the gcc-14-<triplet>-base slices added by the cpp-14 branch.

Both suites bailed out with "Unsupported architecture" there, which fails
outright now that a change to these slices schedules them on all four
runners. The arch lists match what gcc-15 already carries.

The notes pointed at cpp-12-<triplet> for the cc1 compiler, two
generations off. They also carried a TODO saying the architecture-
independent gcc-14 slices do not exist yet, which slices/gcc-14.yaml has
since made untrue.

Replaced with the wording gcc-15 already carries, so the three generations
read the same.

The two new triplet slices follow gcc-15's, including the per-architecture
copyright split the rest of this branch applies to the existing two. Noble
has no gcc-14, so there is nothing to forward port from -- this is canonical#1092
applied to a generation that release never carried.

gcc-14.yaml gains the matching cc1 and gcc-14 entries, so gcc-14_gcc-14 --
the architecture-independent slice that carries /usr/bin/gcc-14 along with
an assembler, a linker and cc1 -- resolves on all four architectures rather
than only amd64 and arm64.

Copies of the gcc-15 suites, which already cover these two runners.

Obsolete since canonical/chisel#256, and its Unsupported architecture
fallback fails outright on the s390x and ppc64le runners -- which a change
to gcc-14.yaml now schedules. Same shape the gcc-15 test already has.