Fix llvm-shlib cmake build

Summary:
This fixes a few things that used to work with a Makefile build, but were broken in cmake.

1. Treat MINGW like a Linux system.
2. The shlib should never contain other shared libraries.

Patch By: Valentin Churavy

Reviewers: axw, beanz

Subscribers: modocache, beanz, mgorny

Differential Revision: https://reviews.llvm.org/D25865

llvm-svn: 285737