Introduce gdbsupport/cxx-thread.h and use it

Introduce gdbsupport/cxx-thread.h and use it

This introduces a new file, gdbsupport/cxx-thread.h, which provides
stubs for the C++ threading functionality on systems that don't
support it.

On fully-working ports, this header just supplies a number of aliases
in the gdb namespace.  So, for instance, gdb::mutex is just an alias
for std::mutex.

For non-working ports, compatibility stubs are provided for the subset
of threading functionality that's used in gdb.  These generally do
nothing and assume single-threaded operation.

The idea behind this is to reduce the number of checks of
CXX_STD_THREAD, making the code cleaner.

Not all spots using CXX_STD_THREAD could readily be converted.
In particular:

* Unit tests
* --config output
* Code manipulating threads themselves
* The extension interrupting handling code

These all seem fine to me.

Note there's also a check in py-dap.c.  This one is perhaps slightly
subtle: DAP starts threads on the Python side, but it relies on gdb
itself being thread-savvy, for instance in gdb.post_event.

Approved-By: Simon Marchi <simon.marchi@efficios.com>