[gdb/testsuite] Fix gdb.btrace/reconnect.exp with native-gdbserver

[gdb/testsuite] Fix gdb.btrace/reconnect.exp with native-gdbserver

When running gdb.btrace/reconnect.exp with native-gdbserver, we run into:
...
FAIL: gdb.btrace/reconnect.exp: first: stepi 19
...
due to the fact that we're trying to match:
...
stepi 19^M
0x00007ffff7dd8b57 in _dl_start () from /lib64/ld-linux-x86-64.so.2^M
...
using pattern:
...
  gdb_test "stepi 19" "0x.* in .* from target.*"
...

Fix this by changing the pattern to:
...
  gdb_test "stepi 19" "0x.* in .* from .*"
...

Tested on x86_64-linux with native and native-gdbserver.

gdb/testsuite/ChangeLog:

2019-04-23  Tom de Vries  <tdevries@suse.de>

	PR gdb/24433
	* gdb.btrace/reconnect.exp: Fix stepi 19 pattern.