Bug 1835189 - Make TARGET_OS in the build system mean the same as target.os in...

Bug 1835189 - Make TARGET_OS in the build system mean the same as target.os in configure. r=firefox-build-system-reviewers,nalexander

The situation is confusing enough.

Eventually, we'll want to have TARGET_FOO mean the same as target.foo,
but let's start with the easy one.

Differential Revision: https://phabricator.services.mozilla.com/D179178