Recognize platform.system() as well as sys.platform

Note: if you are reporting a wrong signature of a function or a class in
the standard library, then the typeshed tracker is better suited
for this report: https://github.com/python/typeshed/issues

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request? feature request
• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

    if platform.system() != "Windows":  # sys.platform != "win32":
        try:
            # os.confstr("CS_GNU_LIBC_VERSION") returns a string like "glibc 2.17".
            version_string = os.confstr("CS_GNU_LIBC_VERSION")
            assert version_string is not None
            _, version = version_string.split()
        except (AssertionError, OSError, ValueError):
            # os.confstr() or CS_GNU_LIBC_VERSION not available (or a bad value)...
            return None
        return version
    return None

• What is the actual behavior/output? "error: Module has no attribute "confstr""
• What is the behavior/output you expect? No error when run under Windows
• What are the versions of mypy and Python you are using? mypy 0.750, Python 3.7.4

Basically it would be nice to get to use platform.system() instead of having to use sys.platform to make mypy happy with code that is conditional on the platform.

Start with the provided Python repro and compare how mypy handles platform.system() with sys.platform in conditional code. Identify the platform-specific analysis entry point and verify the change by checking that os.confstr is not reported as missing in the Windows branch; the payload names no source file or test.