[gdb/testsuite] Fix gdb.threads/threadcrash.exp with glibc debuginfo

[gdb/testsuite] Fix gdb.threads/threadcrash.exp with glibc debuginfo

After installing glibc debuginfo, I ran into:
...
FAIL: gdb.threads/threadcrash.exp: test_live_inferior: \
  $thread_count == [llength $test_list]
...

This happens because the clause:
...
	-re "^\r\n${hs}main$hs$eol" {
...
which is intended to match only:
...
 #1  <hex> in main () at threadcrash.c:423^M
...
also matches "remaining" in:
...
 #1  <hex> in __GI___nanosleep (requested_time=<hex>, remaining=<hex>) at \
   nanosleep.c:27^M
...

Fix this by checking for "in main" instead.

Tested on x86_64-linux.