Prevent snapcraft patching RPATH for library dependencies
By default snapcraft will rewrite the RPATH of binaries to link against
libraries in the core snap. However, the prebuilt binaries used in the
LDC package are designed and expected to link against host system libs.
To allow them to do this we need to explicitly instruct snapcraft using
the `no-patchelf` build attribute.
Switch to using upstream prebuilt binaries for snap package
This is a major (major major MAJOR) change in the package design which
carries 3 major benefits:
* it GREATLY simplifies the snap package design itself, taking
advantage of the existing release packages that are designed
for cross-distro, multi-arch support and reducing snap build
times from hours to seconds
* it allows us to support ARM64 as an install target
* it precisely matches functionality between upstream releases
and the snap package
There are however also some major costs:
* it removes our ability to support i386 install targets, as
upstream does not supply i386 builds
* it assumes that the upstream build settings will match the
needs of the snap package
If this approach is too heavy-handed then it might be possible to make a
compromise such as downloading only prebuilt LLVM and still building the
actual compiler and tool executables.
There seems to be no reason to check out the full git history here and
setting `source-depth: 1` for all git-sourced parts will both speed up
builds and reduce the volume of unnecessary data downloads.
DMD 2.088.x introduced a nasty bug where the expected year in compiler
test output was hardcoded. DMD 2.090.x "fixed" this by updating the
hardcoded year from 2019 to 2020. This patch correspondingly tweaks our
own workaround introduced in d80460670e77175ae0e42ec578b1727726b313db,
which should be possible to remove with LDC 1.21+ (the upstream bug was
fixed properly in DMD 2.091.x).
`ldc2.conf` has been updated to bring it in line with the upstream LDC
build, adding a separate section for 32-bit config and dropping the now
unneeded `--no-warn-mismatch`.
There is no need for this to be a standalone part, as all it does is to
install a couple of build-packages. These can be moved to `ldc`, which
is the only real user, and highlighted as being ctest requirements.
