[RFA 0/8] Ravenscar bug fixes

[Tom Tromey]

This series fixes a few bugs in the Ravenscar thread layer.

For the most part this isn't interesting outside of AdaCore; but this
series has a couple of patches that require review.

Patch #7 is the main one requiring scrutiny.

The bug here is that, when stepping through the context-switching
code, it can appear to gdb that a thread switch occurs in the middle
of the step.  This particular patch works around this problem by
letting gdb pretend that this was intended.  I tried a couple of
approaches to fixing this and this seemed like the cleanest one... but
I would welcome other ideas about how to handle this.

Patch #8 changes the documentation.

Tested internally at AdaCore on various platforms using a Ravenscar
runtime.

Tom