Disable `perf` support in the profiler when running on Android (for now).

We need to assess the situation before we enable this everywhere. Apparently
some older devices have trouble with `perf`.
Merge branch 'master' of github.com:mono/mono
Mark the `spin` variable in atomic.c as G_GNUC_UNUSED.

It would require an `#ifdef` forest to get the conditions
under which it is used right.
Bring the implementation of the 64-bit CAS fallback in line with the 32-bit one.