gdb/testsuite: Add mechanism to compile Windows native programs on Cygwin

This adds a new "win32" option to gdb_compile that lets us compile
native Windows programs when testing Cygwin.  Will be used in a
following patch.

The testsuite tries to find the compiler to use automatically.  You
can also explicitly set it with WIN32_CC_FOR_TARGET.

Change-Id: I86e954494cf7f88164cdf1c9127be46d5437986f