Separate the eglib source and build instructions

Separate the eglib source and build instructions

This change extracts the eglib source files from the build instuctions
for builds via MSVC on Windows. It also separates the source files into
platform-specific and common .targets files. This provides more
flexibility for builds from MSVC in a few important ways:

1. We can do MSVC-based builds for platforms that will not be pushed to
public repos by checking out the Mono code externally and importing the
.targets files into a custom .vcxproj for the platform.
2. We can replace the platform specific .targets files with a custom set
of source files to implement a different platform abstraction layer
that will not be public without the need to merge changes to the
.vcxproj from the public repo.

This allows for ease of synchronizing changes to the public Mono
repo with private forks, while _not_ changing the behavior of the code
in the public repo.