[gdb/testsuite] Fix probe test in gdb.base/break-interp.exp

[gdb/testsuite] Fix probe test in gdb.base/break-interp.exp

When running test-case gdb.base/break-interp.exp on ubuntu 18.04.5, we have:
...
 (gdb) bt^M
 #0  0x00007eff7ad5ae12 in ?? () from break-interp-LDprelinkNOdebugNO^M
 #1  0x00007eff7ad71f50 in ?? () from break-interp-LDprelinkNOdebugNO^M
 #2  0x00007eff7ad59128 in ?? () from break-interp-LDprelinkNOdebugNO^M
 #3  0x00007eff7ad58098 in ?? () from break-interp-LDprelinkNOdebugNO^M
 #4  0x0000000000000002 in ?? ()^M
 #5  0x00007fff505d7a32 in ?? ()^M
 #6  0x00007fff505d7a94 in ?? ()^M
 #7  0x0000000000000000 in ?? ()^M
 (gdb) FAIL: gdb.base/break-interp.exp: ldprelink=NO: ldsepdebug=NO: \
         first backtrace: dl bt
...

Using the backtrace, the test-case tries to establish that we're stopped in
dl_main.

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.

As in "[gdb/testsuite] Improve probe detection in gdb.base/break-probes.exp",
fix this by doing info probes and grepping for the address.

Tested on x86_64-linux.