[Package] Add a potentially necessary linked library on Linux.
[Package] Add a potentially necessary linked library on Linux.

 - In some cases, the LLVM support library ends up needing to be linked against
   libm.

apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild,apple/swift-llbuild