Merge pull request #2794 from xmcclure/win-aot-basic

Make AOT compilation on 64 bit windows produce executables

This PR contains three commits (see commit descriptions) which:

- Fix 64-bit debug windows builds in MSVC 2015
- Make it possible to use the tool-prefix aot option with mono/mini make fullaotcheck (necessary if you are in cygwin and wish to invoke `x86_64-w64-mingw32-gcc`)
- Change mini so that `--aot=full` successfully emits 64-bit windows executables.

Building 32 bit executables will generally fail because we do not follow cdecl. The 64 bit executables do not run properly when executed.

diff --cc mono/mini/aot-compiler.c
Simple merge

diff --cc mono/mini/image-writer.c
Simple merge