[cmake] Append -Wl,-rpath-link conditionally to GNULD

Append -Wl,-rpath-link conditionally to whether GNU ld.bfd is used
rather than the Linux+!gold conditionals. Also move it out of 'else'
branch of *BSD handling. This fixes build failures with ld.bfd
on Gentoo/FreeBSD, and should cause no harm on other systems using
ld.bfd.

This patch improves the original logic by reusing results of linker
detection introduced in r307852.

Differential Revision: https://reviews.llvm.org/D43751

llvm-svn: 327007