Adjust gdb.thread/pthreads.exp for Cygwin

Adjust gdb.thread/pthreads.exp for Cygwin

The Cygwin runtime spawns a few extra threads, so using hardcoded
thread numbers in tests rarely works correctly.  Thankfully, this
testcase already records the ids of the important threads in globals.
It just so happens that they are not used in a few tests.  This commit
fixes that.

With this, the test passes cleanly on Cygwin [1].  Still passes cleanly on
x86-64 GNU/Linux.

[1] - with system GDB.  Upstream GDB is missing a couple patches
Cygwin carries downstream.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I01bf71fcb44ceddea8bd16b933b10b964749a6af