[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.

gdb/testsuite/gdb.base/break-interp.exp

@@ -84,6 +84,16 @@ gdb_test_multiple $test $test {
 }
 
 set interp_system [section_get [standard_output_file $binfile_test] .interp]
+clean_restart $interp_system
+set dl_main_found 0
+gdb_test_multiple "info addr dl_main" "" {
+    -re -wrap "Symbol \"dl_main\" is a function at address $hex\\." {
+	set dl_main_found 1
+    }
+    -re -wrap "No symbol \"dl_main\" in current context\\." {
+    }
+}
+
 set interp_system_debug [system_debug_get $interp_system]
 verbose -log "$interp_system has debug $interp_system_debug"
 
@@ -381,6 +391,7 @@ proc test_attach {file displacement {relink_args ""}} {
 
 proc test_ld {file ifmain trynosym displacement} {
     global srcdir subdir gdb_prompt expect_out inferior_exited_re solib_bp
+    global hex dl_main_found
 
     # First test normal `file'-command loaded $FILE with symbols.
 
@@ -413,7 +424,28 @@ proc test_ld {file ifmain trynosym displacement} {
 
     with_test_prefix "first backtrace" {
 	gdb_test_no_output "set verbose off"
-	gdb_test "bt" "#0 +\[^\r\n\]*\\m(__GI_)?$solib_bp\\M.*" "dl bt"
+	gdb_test_multiple "bt" "dl bt" {
+	    -re -wrap "#0 +\[^\r\n\]*\\m(__GI_)?$solib_bp\\M.*" {
+		pass $gdb_test_name
+	    }
+	    -re -wrap "#0  ($hex) .*" {
+		if { $dl_main_found } {
+		    fail $gdb_test_name
+		} else {
+		    set pc $expect_out(1,string)
+		    regsub "0x0*" $pc "" pc
+		    gdb_test "p /x 0x$pc" ".*"
+		    gdb_test_multiple "info probes stap rtld" $gdb_test_name {
+			-re -wrap "(?:init_start|init_complete) +0x0*$pc .*" {
+			    pass $gdb_test_name
+			}
+			-re -wrap "" {
+			    fail $gdb_test_name
+			}
+		    }
+		}
+	    }
+	}
 	gdb_test_no_output "set verbose on"
     }