AutootolsToolchain: pass a valid GNU triplet when the compiler is msvc

[jcar87]

Changelog: Bugfix: Fix bug to pass a valid GNU triplet when using AutotoolsToolchain and cross-building on Windows.
Docs: Omit

Close: #7460

Context:

The current implementation of the private _get_gnu_triplet returns x86_64-windows-msvc. This results in errors when we are cross-building on Windows, since this is not a valid GNU triplet:

config.sub x86-64-windows-msvc    
Invalid configuration `x86-64-windows-msvc': OS `msvc' not recognized

Specifically for Autotools, the triplet system type is of the format specified here, where we can have:

• cpu-company-os, or
• cpu-company-kernel-os

In this context, msvc is not a valid OS. There is no current convention in GNU for a proper target when the OS is Windows, and the C/C++ runtime are provided by Visual C++, this PR falls back to a generic {arch}-unknown-windows, where windows is a valid OS. This is the a very generic fallback - while x86-64-w64-mingw32 may be preferred in some contexts, we don't have enough context to assume it (but could be done in a separate flag), and when Visual C++ with the Microsoft Visual C++ runtime is used, the binaries produced would not strictly be mingw32 binaries.