[gdb/testsuite] Add gdb.opt/break-on-_exit.exp

[gdb/testsuite] Add gdb.opt/break-on-_exit.exp

Add a test-case to excercise the problem scenario reported in PR28527 and
fixed in commit a50bdb99afe "[gdb/tdep, rs6000] Don't skip system call in
skip_prologue":
- set a breakpoint on _exit, and
- verify that it triggers.

Note that this is not a regression test for that commit.  Since the actual
code in _exit may vary across os instances, we cannot guarantee that the
problem will always trigger with this test-case.

Rather, this test-case is a version of the original test-case
(gdb.threads/process-dies-while-detaching.exp) that is minimal while still
reproducing the problem reported in PR28527, in that same setting.

The benefit of this test-case is that it exercise real-life code and may
expose similar problems in other settings.  Also, it provides a much easier
test-case to investigate in case a similar problem occurs.

Tested on x86_64-linux and ppc64le-linux.