[gdb/testsuite] Improve probe detection in gdb.base/break-probes.exp

[gdb/testsuite] Improve probe detection in gdb.base/break-probes.exp

When running test-case gdb.base/break-probes.exp on ubuntu 18.04.5, we have:
...
 (gdb) run^M
 Starting program: break-probes^M
 Stopped due to shared library event (no libraries added or removed)^M
 (gdb) bt^M
 #0  0x00007ffff7dd6e12 in ?? () from /lib64/ld-linux-x86-64.so.2^M
 #1  0x00007ffff7dedf50 in ?? () from /lib64/ld-linux-x86-64.so.2^M
 #2  0x00007ffff7dd5128 in ?? () from /lib64/ld-linux-x86-64.so.2^M
 #3  0x00007ffff7dd4098 in ?? () from /lib64/ld-linux-x86-64.so.2^M
 #4  0x0000000000000001 in ?? ()^M
 #5  0x00007fffffffdaac in ?? ()^M
 #6  0x0000000000000000 in ?? ()^M
 (gdb) UNSUPPORTED: gdb.base/break-probes.exp: probes not present on this system
...

Using the backtrace, the test-case tries to establish that we're stopped in
dl_main, which is used as proof that we're using probes.

However, the backtrace only shows an address, because:
- the dynamic linker contains no minimal symbols and no debug info, and
- gdb is build without --with-separate-debug-dir so it can't find the
  corresponding .debug file, which does contain the mimimal symbols and
  debug info.

Fix this by instead printing the pc and grepping for the value in the
info probes output:
...
(gdb) p /x $pc^M
$1 = 0x7ffff7dd6e12^M
(gdb) info probes^M
Type Provider Name           Where              Object                      ^M
  ...
stap rtld     init_start     0x00007ffff7dd6e12 /lib64/ld-linux-x86-64.so.2 ^M
  ...
(gdb)
...

Tested on x86_64-linux.