[msvc] Change target MSVC release from v120 to v120_xp

[msvc] Change target MSVC release from v120 to v120_xp

They're basically the same thing, but v120 has DX10+ capabilities and v120_xp
is DX9 only. Since we're not building against DirectX, the difference is
purely academic - but changing the value to v120_xp means MSVC binaries run
on XP+, not Vista+