Give thread names to ease debugging

[linkmauve]

This makes it much more easy to figure out why a given thread is consuming CPU time for instance.

This is still missing sound threads, but those depend on SDL apparently.

[zeromus]

update rthreads from libretro-common before revising it

[linkmauve]

It is now merged. How do you usually deal with synchronising your vendor’d dependencies?

[zeromus]

paste from vendor and check diffs to make sure there are not important local changes clobbered

[zeromus]

what's the deal with this? have are we synchronized with libretro-common yet?

[zeromus]

let's merge this and if it breaks the build, someone will have to update rthreads

[MaddTheSane]

…And breaks macOS: Mac's pthread_setname_np takes only one argument: the new thread name. And it can only be called from the thread you want to rename.

A quick and dirty fix is in #422.