Make separate config variable for `LLVM_LIBRARY_DIRS`

@beanz tells me it should in fact exists, and it would be bad to put a
list of dirs in `LLVM_LIBRARY_DIR`. I am therefore making some changes
to help avoid this an other mistakes:

- Use a separate `LLVM_CONFIG_LIBRARY_DIR` (no "S") variable so we don't
  start putting a list in `LLVM_LIBRARY_DIR` by mistake.

- Define the individual dir variables first, and the define the list of
  dirs variables programmatically.

Also, I rearranged the definitions of the "regular" single dirs below so
`LLVM_BINARY_DIR`, which is the unsuffixed one, comes first before the
suffixed ones.

Reviewed By: beanz

Differential Revision: https://reviews.llvm.org/D116497