[bolt][llvm][cmake] Use `CMAKE_INSTALL_LIBDIR` too

Working back towards D130586.

Bolt didn't use `LLVM_LIBDIR_SUFFIX` before, and has no in-tree reverse
dependencies, it seems easier to add.

The change in LLVM itself is to prevent some unexpected `lib64` from
cropping up due to the `CMAKE_INSTALL_LIBDIR` defaulting logic.

Differential Revision: https://reviews.llvm.org/D132297