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

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

With gdb.gdb/selftest.exp, we get:
...
(xgdb) PASS: gdb.gdb/selftest.exp: send SIGINT signal to child process
^M
Thread 1 "xgdb" received signal SIGINT, Interrupt.^M
0x00007ffff5bf01db in poll () from /lib64/libc.so.6^M
(gdb) FAIL: gdb.gdb/selftest.exp: send ^C to child process again
...

The failure is due to gdb printing 'Thread 1 "xgdb" received signal SIGINT',
but the regexp in the test-case expecting 'Program received signal SIGINT'.

Fix this by updating the regexp, similar to how that is done earlier in the
test-case.

gdb/testsuite/ChangeLog:

2019-08-13  Tom de Vries  <tdevries@suse.de>

	* gdb.gdb/selftest.exp (send ^C to child process again): Accept also
	Thread.