gdb/testsuite: Fix gdb.threads/threadcrash.exp with check-readmore

gdb/testsuite: Fix gdb.threads/threadcrash.exp with check-readmore

With check-readmore, I run into:
...
FAIL: gdb.threads/threadcrash.exp: test_corefile: \
  $thread_count == [llength $test_list]
...

The problem is that the clauses in the gdb_test_multiple for
"thread apply all backtrace" intent to match one line, but actually can
match more than one line, and consequently a match for one type of thread can
consume a line that was supposed to match another thread.

For instance, there's this regexp:
...
	    -re "\[^\n\]*syscall_task .location=SIGNAL_ALT_STACK\[^\n\]*" {
...

It's limited at the end by \[^\n\]*, meaning the match stops at the end of the
line.

But it doesn't start with a ^, and consequently can match more than one line.
The "\[^\n\]*" at the start doesn't prevent this, there's an implicit .* at
the start of each pattern, unless it's anchored using a ^.

Fix this by rewriting the regexps in a "^\r\n$hs$regexp$hs$eol" style, where:
- hs is: \[^\n\]* (horizontal space), and
- eol is (?=\r\n) (look-ahead end-of-line).

It also turned out to be necessary to drop the -lbl switch, and introduce a
corresponding explicit clause.  The -lbl clause is placed ALAP, and
consequently allowed the default fail clause to trigger.

Tested on arm-linux and x86_64-linux.