Abi::hostAbi(): Add MSVC2015.

Silence startup warning:
Unable to completely determine the host ABI (x86-windows-unknown-pe-64bit).

Change-Id: I89aae738d25d7c97f352172c98628ddd0987d6dc
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>