Update chpl_platform.py to support cygwin32 and cygwin64.

Cygwin now supports both 64bit and 32bit versions. Update chpl_platform.py to
distinguish between 32bit and 64bit in the same way linux32 and linux64.

Tests and documentation will need to updated to reflect the new name.
Add Makefile.cygwin{32,64} files that just include Makefile.cygwin.
Move scope of machine up to be more accessible in chpl_platform.py.
Fix makefile include paths.
Update cygwin comparisons in util/chplenv/
Update testRelease to use chpl_platform.py.

testRelease was duplicating some of the logic for detecting platform, so just use chpl_platform.py.

Also fix comparison against cygwin in nightly.
Add symlinks for runtime/include/cygwin{32,64}/.

Both cygwin32 and cygwin64 have symlinks to runtime/include/cygwin/.
Fix typo: x64_64 -> x86_64
Add copyrights to new makefiles.
Update docs and .skipifs for new cygwin platform vals.
Fix termination condition for getNextExpr
Merge pull request #959 from noakesmichael/getFirstAndNext

Fix termination condition for getNextExpr

I transformed getFirstExpr/getNextExpr from a pair of functions that dispatch on BaseAST to
a pair of virtual methods on BaseAST; this was done to allow for expansion to support While/For
loops.  However I screwed up the termination condition on getNextExpr which caused a "fence post"
error when iterating over certain expression types.

This error was not surfaced in any current code/tests but new code by Elliot caught my error.

Compiled: STD clang/darwin, STD gcc/linux64, LLVM gcc/linux64
Tested: paratest --verify
Reviewed: Extensive email thread with Elliot
Merge pull request #955 from thomasvandoren/cygwin-platforms

Distinguish between 32bit and 64bit cygwin platforms.

Add support for `cygwin32` and `cygwin64` platform values, and retire
having the one `cygwin` value. Update runtime, makefiles, test .skipifs,
and docs for the new values.

Cygwin has supported both 32- and 64-bit for some time. They can be
installed side-by-side on a single system. This update allows Chapel to
support both cygwin architectures.

I verified this works on two cygwin boxes, one with cygwin-x86 (aka
platform=cygwin32) and the other with cygwin-x86_64 (aka
platform==cygwin64). I ran nightly against examples just to make sure
nothing was fundamentally broken. I also ran
`start_test --futures test/io/ferguson/utf8/` on cygwin32, cygwin64, and
darwin and verified that .skipifs in that dir worked as expected.

r: @Kyle-B