Major improvement of Visual Studio toolchain and Windows SDK support in mkbundle.

Major improvement of Visual Studio toolchain and Windows SDK support in mkbundle.

Current implementation of Windows builds in mkbundle is not supporting
Visual Studio toolchains and Windows SDK's in a working way. Windows version
of mkbundle also lacked support for 64-bit builds together with a dependency on
GCC assembler etc.

This commit improves things in many different areas. The following toolchains
are intended to work together with this commit:

VS2013, VS2015, VS2017.

NOTE, default Mono SDK distribution is build targeting VS2015/VS2017 and static
version of libmono-static.lib won’t work with VS2013. If static use case is needed
with VS2013, a custom build mono under VS2013 must be used.  Dynamic use case is
working as expected for all three versions.

The commit also improves the support for Cygwin GCC compatible toolchains on Windows.

The commit adds support to use installed versions of both Windows 8.1 as well
as Windows 10 SDK's together with all above VS toolchains.

The commit extends support to include 64-bit mkbundle builds (using all supported
Windows toolchains). The target is decided based on mkbundle process type, 32-bit
mono builds 32-bit bundle and 64-bit mono builds 64-bit bundle.

mkbundle might still need to have a working GCC as.exe when running, but this commit
also adds support for Clang for VS CodeGen as assembler available in VS2015 Update3
and VS2017 (separate option in VS setup). When using this option there is no needed
for available GCC toolchains when running mkbundle on Windows.

A recommended way of running mkbundle on Windows is by using one of the targeted toolchains
development consoles. This way mkbundle will get all needed paths and installation folders
directly from the development consoles environment, matching a predefined setup.

Old mkbundle environment variables on Windows will continue to work. There are also a number
of new variables that can be set (or are set by the VS development consoles). The important ones
are:

VisualStudioVersion: set to pick a specific VS version, like 12.0, 14.0 or 15.0. If a none existing
version is specified, mkbundle will fallback to latest installed version.

WindowsSdkVersion: set to pick a specific Windows SDK version, like 8.1, 10.0.10240.0, 10.0.15063.0 etc.
If a none existing version is specified, mkbundle will fallback to latest installed version.

AS: If Clang for VS CodeGen is not installed this must point to a working as.exe assembler.

MONOPREFIX: set to pick a custom Mono SDK install root directory (need to match the architecture of mkbundle
process running). If not set, mkbundle will look for installed Mono SDK’s matching targeted architecture.

MONOLIB: if a different library name is used, it can be specified here. It is also possible to add an absolute
path to the mono library used in linking. NOTE, correct library needs to be supplied for static/dynamic use cases.

INCLUDE: override all custom include path handling, normally set by VS developer consoles.

LIB: override all custom lib path handling, normally set by VS developer consoles.

VCCRT: Gives possibility to use static or dynamic C-Runtime linking. By default mkbundle will use dynamic C-Runtime
linking on Windows to be compatible with default Mono SDK distribution. If a custom build Mono
using static C-Runtime linkage is used, setting this variable to MT (default MD), will link using static
C-Runtime.

VCSUBSYSTEM: mkbundle will by default use windows subsystem. If console subsystem is preferred this variable
can be set to console.

Should resolve:

https://bugzilla.xamarin.com/show_bug.cgi?id=57893

and partial resolves (needs https://github.com/mono/mono/pull/5656 as well):

https://bugzilla.xamarin.com/show_bug.cgi?id=57892