[cmake] Support LLD for CLANG_ORDER_FILE

LLD also supports order files using the `--symbol-ordering-file` option.
As the name would suggest, the order file format is slightly different
from gold; gold's order files specify section names, whereas LLD's
specify symbol names. Assuming you have an order file in the correct
format though, we should support using it with LLD.

Switch the check to actually use LLVM's linker detection rather than
just checking for the presence of the gold executable, since we might
have a gold executable present but be using LLD (or bfd for that matter)
as our linker.

Differential Revision: https://reviews.llvm.org/D47669

llvm-svn: 333810