win,build: support Visual C++ Build Tools 2015

Pull Request check-list

• Does make -j8 test (UNIX) or vcbuild test nosign (Windows) pass with this change (including linting)?
• Is the commit message formatted according to [CONTRIBUTING.md][0]?

Affected core subsystem(s)

win, build

Description of change

To build with Microsoft Visual C++ Build Tools 2015 Technical Preview Update 2 RC, msbuild should be invoked with a parameter specifying the platform (/p:Platform=). This is necessary because vcvarsall.bat sets a default platform that is not present in the solution generated by Gyp, failing the build.

CI machines run a previous version of VCBT, with a version of vcvarsall.bat modified to be able to build node. With the updated version in Update 2, only this change is needed to build with unmodified VCBT.

This is a trivial cherry-pick for v4-staging, v0.12-staging and v0.10-staging.

cc @nodejs/platform-windows