[2019-02][sdks] Build Android SDK on Windows (#14472)

[2019-02][sdks] Build Android SDK on Windows (#14472)

* [sdks] allow building on cygwin

* [sdks] Cherrypick platform detection from f83d9f3575776a5c7edad336f21f5c70ce519bc4

* [sdks] Start adding stub templates to llvm.mk

* [sdks] provision-android stubs for win32

* [sdks] Pass cygwin host triple to calls to configure on win32

* [sdks] Add runtime stub templates

* [sdks] Add android stub templates for win32

This is enough to get configure-android to work on cygwin x86_64

* [sdks] On win32, the HostMxe build is the host buildwe want the x86_64-w64-mingw32 toolchain, not the cygwin toolchain

* [sdks] For android on cygwin depend on the mingw64-{x86_64,i686}-zlib

* [sdks] Add android SDK build artifacts to .gitignore

* [sdks] Add AndroidCrossMXETemplate stub to android.mk for Windows builds

* [sdks] Fixup comments in android.mk

* [sdks] use UNAME=Windows for Cygwin and WSL

Don't assume Cygwin is the build platform for Windows Android SDK builds.

Keep using UNAME for coarse platform descriptions.
Switch to HOST_ARCH_MINGW32 instead of CYGWIN_BUILD_MACHINE for variable holding
the machine part of the host triplet for Windows.

* [sdks] Dummy provision-mxe target for windows

* [ci] Don't provision-android on Windows

It doesn't work yet.

* [sdks] temporarily turn off android-host-mxe-Win64-release

Needs mingw64-x86_64-zlib on all the CI builds bots.

* FIXME: hack - don't set MONO_EXECUTABLE to the msvc mono for Android SDK build

* [sdks] drop LLVMProvisionTemplateStub