libc variable should default to unknown for non-linux target_platform?

Nobody has claimed this yet.

## TLDR;
I claim that the `libc` variable makes no sense for windows targets and should default to "unknown" when installing prebuilts on windows _and_ when installing prebuilts for windows on linux.

## Long Story
When invoking node-pre-gyp from win32, `libc` will default to the value "unknown".
When invoking node-pre-gyp from a linux platform (i.e. docker), but targeting win32, `libc` will default to whatever value is detected for the host. Thus, if a native package includes `libc` within the `package_name` template, node-pre-gyp will attempt to download the wrong prebuilt binary.

A real world example of "installing prebuilts _for_ windows _on_ linux": electron-builder allows one to build a windows app distribution from a docker instance if all native modules have win32 prebuilts available. This mostly works, except for the case where the native module uses the `libc` variable within the `package_name` template. An example of such a package is `grpc-node`, which is used by many packages such as `firebase` for example.

Thus, should node-pre-gyp default the `libc` variable to unknown when targeting win32?

Or, is it the responsibility of each and every client of node-pre-gyp to ensure that the `target_libc` option is set to "unknown" for the edge case of cross-installing dependencies?

Start by tracing how node-pre-gyp determines libc and target_platform during native package installation, including the package_name template and target_libc option. Reproduce installation for a win32 target from both win32 and Linux hosts. Done means win32-targeted prebuilts consistently use libc as unknown, with coverage for both host scenarios.

Written by the indexing model from the issue text.