Fix Ravenscar "process" resume

A coworker noticed that gdb would send the wrong vCont packet to qemu
when debugging a Ravenscar program:

    > (gdb) thread 2
    > [Switching to thread 2 (Thread 1.2)]
    > #0  0x0000000000001000 in ?? ()

    > (gdb) c
    [...]
    > Sending packet: $vCont;c:p1.1#e2...Ack

Here, we've switched to thread 2, but the packet says to resume thread
1.

This turned out to be a bug in ravenscar_thread_target::resume, which
did not properly handle the case of a "process" resume.  In
particular, the resume method would be passed a ptid of (1, 0, 0) --
but then rewrite this to its saved ptid.

This patch fixes the problem by recognizing this case in the resume
method.

gdb/ChangeLog
2020-08-07  Tom Tromey  <tromey@adacore.com>

	* ravenscar-thread.c (ravenscar_thread_target::resume): Handle
	"is_pid" case.