Adjust gdb.base/sigall.exp for Cygwin

Adjust gdb.base/sigall.exp for Cygwin

The gdb.base/sigall.exp testcase has many FAILs on Cygwin currently.

From:

 Thread 1 "sigall" received signal SIGPWR, Power fail/restart.
 0x00007ffeac9ed134 in ntdll!ZwWaitForSingleObject () from /cygdrive/c/Windows/SYSTEM32/ntdll.dll
 (gdb) FAIL: gdb.base/sigall.exp: get signal LOST

we see two issues.  The test is expecting "Program received ..." which
only appears if the inferior is single-threaded.  All Cygwin inferiors
are multi-threaded, because both Windows and the Cygwin runtime spawn
a few helper threads.

And then, SIGLOST is the same as SIGPWR on Cygwin.  The testcase
already knows to treat them the same on SPARC64 GNU/Linux.  We just
need to extend the relevant code to treat Cygwin the same.

With this, the test passes cleanly on Cygwin.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Ie3553d043f4aeafafc011347b6cb61ed58501667