[llvm] llvm-shlib: Fix mingw dll exports (PR #148772)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

Oh, also: Before merging, I'd suggest removing the CC mention in the PR description, according to https://discourse.llvm.org/t/forbidding-username-in-commits/86997.

Also, the 21.x branch is being made probably within the next couple of hours, so to save some effort, I'd suggest merging it sooner rather than later. :-)

https://github.com/llvm/llvm-project/pull/148772