[gdb/testsuite] Fix gdb.gdb/selftest.exp

[gdb/testsuite] Fix gdb.gdb/selftest.exp

With a gdb build with CFLAGS "-O2 -g -flto=auto", I run into:
...
 #7  gdb_main (args=0x7fffffffd220) at src/gdb/main.c:1368^M
 #8  main (argc=<optimized out>, argv=<optimized out>) at src/gdb/gdb.c:32^M
 (gdb) FAIL: gdb.gdb/selftest.exp: backtrace through signal handler
...
which means that this regexp in proc test_with_self fails:
...
  -re "#0.*(read|poll).*in main \\(.*\\) at .*gdb\\.c.*$gdb_prompt $" {
...

The problem is that gdb_main has been inlined into main, and consequently the
backtrace uses:
...
 #x  <fn> ...
...
instead of
...
 #x  <address> in <fn> ...
...

Fix this by updating the regexp to not require "in" before " main".

Tested on x86_64-linux.