This commit adds a new exception 'gdb_exception_forced_quit', reason
code 'REASON_FORCED_QUIT', return mask 'RETURN_MASK_FORCED_QUIT', and
a wrapper for throwing the exception, throw_forced_quit().

The addition of this exception plus supporting code will allow us to
recognize that a SIGTERM has been received by GDB and then propagate
recognition of that fact to the upper levels of GDB where it can be
correctly handled.  At the moment, when GDB receives a SIGTERM, it
will attempt to exit via a series of calls from the QUIT checking
code.  However, before it can exit, it must do various cleanups, such
as killing or detaching all inferiors.  Should these cleanups be
attempted while GDB is executing very low level code, such as reading
target memory from within ps_xfer_memory(), it can happen that some of
GDB's state is out of sync with regard to the cleanup code's
expectations.  In the case just mentioned, it's been observed that
inferior_ptid and the current_thread_ are not in sync; this triggers
an assert / internal error.

This commit only introduces the exception plus supporting machinery;
changes which use this new exception are in later commits in this
series.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=26761


Tested-by: Tom de Vries <tdevries@suse.de>
Approved-by: Pedro Alves <pedro@palves.net>