[gdb] Fix info checkpoints

[gdb] Fix info checkpoints

Consider test-case gdb.base/checkpoint.exp.  At some point, it issues an info
checkpoints command:
...
(gdb) info checkpoints^M
* 0 process 30570 (main process) at 0x0^M
  1 process 30573 at 0x4008bb, file checkpoint.c, line 49^M
  2 process 30574 at 0x4008bb, file checkpoint.c, line 49^M
  3 process 30575 at 0x4008bb, file checkpoint.c, line 49^M
  4 process 30576 at 0x4008bb, file checkpoint.c, line 49^M
  5 process 30577 at 0x4008bb, file checkpoint.c, line 49^M
  6 process 30578 at 0x4008bb, file checkpoint.c, line 49^M
  7 process 30579 at 0x4008bb, file checkpoint.c, line 49^M
  8 process 30580 at 0x4008bb, file checkpoint.c, line 49^M
  9 process 30582 at 0x4008bb, file checkpoint.c, line 49^M
  10 process 30583 at 0x4008bb, file checkpoint.c, line 49^M
...

According to the docs, each of these (0-10) is a checkpoint.

But the pc address (as well as the file name and line number) is missing for
checkpoint 0.

Fix this by sampling the pc value for the current process in
info_checkpoints_command, such that we have instead:
...
* 0 process 30570 (main process) at 0x4008bb, file checkpoint.c, line 49^M
...

Tested on x86_64-linux.

Approved-By: Kevin Buettner <kevinb@redhat.com>
PR gdb/31211
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31211