[CMake] Allow for Solaris ld -V output on stdout

In recent versions of Solaris 11.4 (previously 12), ld -V output went to
stdout instead of stderr. Since AddLLVM.cmake only expects it on stderr,
Solaris ld wasn't properly detected and options not understood by it are
passed during the build.

The following patch fixes this, allowing for both variants.

Tested on i386-pc-solaris2.11.4 (on top of D35755 which is needed for
proper Solaris support).

Patch by Rainer Orth.

Differential Revision: https://reviews.llvm.org/D39601

llvm-svn: 318532