[llvm] prefix linker flag on non-MSVC compilers with `-Wl,`

Prior to this patch, a Windows build of llvm-lto using clang failed with
the error: `LTO.def: unknown file type`.  The reason for this failure is
that .DEF files are used by the linker not by the clang compiler.  The
MSVC compiler+linker handles this transparently, but if we're using
clang (or gcc), then we need to tell the compiler to forward this flag
to the linker. This patch adds the necessary `-Wl` flag to fix the
problem.

Reviewed By: rnk, mstorsjo

Differential Revision: https://reviews.llvm.org/D134165