Run bootstrapping tests on Buildkite

[wilzbach]

Once this is working, it would allow to retire SemaphoreCi.
OTOH the four parallel builds we get at SemaphoreCi are really fast, so maybe we don't want to do this until we have more agents or flexible scaling.

TODO for the future:

• Fix 32-bit failures (install gcc-multilib + libcurl3-dev:i386)
• Fix failures when running test in druntime (due to a too old gdb on Debian)
• Don't block on the bootstrapping tests (e.g. include them from the wait)
• Install GDC locally (currently it's installed as Ubuntu apt repository)

[dlang-bot]

Thanks for your pull request, @wilzbach!

[wilzbach]

So ideally we could start these tests in the beginning (they are totally independent to the distribution step and the following projects.)
However, it looks like a wait is a global one and thus we seem to only have two options:

• add them after the wait on the distribution
• add the bootstrapping projects as separate pipeline

[wilzbach]

Rebased and changed, s.t. for now only DMD x86_64 + LDC x86_64 bit will be tested. Also as gdb 8 is missing on the agents, a few tests fail. I removed them for now in a similar way to #291 and #301.
We still need to figure out a way to fetch gdc binaries which can be used locally.

[wilzbach]

Looks like it's failing with LDC:

../generated/linux/release/64/backend.a(evalu8.o):evalu8.d:function evalu8(elem*, unsigned int): error: undefined reference to '_Z7_moduloeS_'
--
  | ../generated/linux/release/64/backend.a(evalu8.o):evalu8.d:function evalu8(elem*, unsigned int): error: undefined reference to '_Z7_moduloeS_'
  | ../generated/linux/release/64/backend.a(evalu8.o):evalu8.d:function evalu8(elem*, unsigned int): error: undefined reference to '_Z7_moduloeS_'