CUDA/x86_64 offloading Buildbot

Hi folks,

the buildbot for testing OpenMP offloading (CUDA and x86_64) is now on
Staging (Buildbot). In order to
become a production buildbot such that blame mails are sent if
something breaks, the buildbot first has to become green for a couple
of days.

Unfortunately, the x86_64 plugin has two bugs that need to be fixed:

1) x86_64-pc-linux-gnu::memory_manager.cpp and

fails with

int main(int, char **): Assertion `buffer[j] == i' failed.

If really necessary, these could be marked with `XFAIL: x64_64` but
preferably, we fix this.

2) x86_64-pc-linux-gnu::bug49334.cpp

This is a fragile test that only sometimes fails with

    note: command had no output on stdout or stderr

Again, if, nobody is going to fix it, we could use an UNSUPPORTED as
already the case with amdgcn-amd-amdhsa.