[llvm-shlib] Do not use version script when building with MinGW
The MinGW driver for lld does not support the --version-script option.
For GNU ld, it's a no-op since LLVM.dll exports all symbols.
Reviewers: srhines, mstorsjo
Subscribers: mgorny, fedor.sergeev, llvm-commits
Differential Revision: https://reviews.llvm.org/D63743