Fix thread target ID of exited waves

Fix thread target ID of exited waves

Currently, if you step over kernel exit, you see:

 stepi
 [AMDGPU Wave ?:?:?:1 (?,?,?)/? exited]
 Command aborted, thread exited.
 (gdb)

Those '?' are because the thread/wave is already gone by the time GDB
prints the "exited" notification, we can't ask dbgapi for any info
about the wave anymore.

This commit fixes it by caching the wave's coordinates as soon as GDB
sees the wave for the first time, and making
amd_dbgapi_target::pid_to_str use the cached info.

At first I thought of clearing the wave_info object from a
thread_exited observer.  However, that is too soon, resulting in this:

 (gdb) si
 [AMDGPU Wave 1:4:1:1 (0,0,0)/0 exited]
 Command aborted, thread exited.
 (gdb) thread
 [Current thread is 6 (AMDGPU Wave ?:?:?:0 (?,?,?)/?) (exited)]

We need instead to clear the wave info when the thread is ultimately
deleted, so we get:

 (gdb) si
 [AMDGPU Wave 1:4:1:1 (0,0,0)/0 exited]
 Command aborted, thread exited.
 (gdb) thread
 [Current thread is 6 (AMDGPU Wave 1:4:1:1 (0,0,0)/0) (exited)]

And for that, we need a new thread_deleted observable.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Approved-By: Lancelot Six <lancelot.six@amd.com> (amdgpu)
Change-Id: I6c3e22541f051e1205f75eb657b04dc15e547580