Add support for profiling the compiler with Samply.

[nnethercote]

I just tried out Samply, it works on rustc and seems like a nice addition.

r? @Kobzol

[Kobzol]

I tried it locally. Very nice visualisation of the stacks! A nice addition to the profiler suite.

Could you just add quick a smoke test for it in ci/check-profiling.sh? It should be also installed on CI, in the part where we install cargo-llvm-lines.

[nnethercote]

The test fails with:

Failed to start profiling: Operation not permitted (os error 1)

I think Samply can't be tested on CI for the same reason as perf :(

[nnethercote]

I updated, removing the test, but adding a comment to ci/check-profiling.sh explaining why Samply isn't tested.