Add option LLVM_NM to allow specifying the location of the llvm-nm tool

The new option works like the existing LLVM_TABLEGEN, and
LLVM_CONFIG_PATH options.  Instead of building llvm-nm, the build uses
the executable defined by LLVM_NM.

This is useful for cross-compilation scenarios where the host cannot run
the cross-compiled tool, and recursing into another cmake build is not
an option (due to required DEFINE's, for example).

Reviewed By: smeenai

Differential Revision: https://reviews.llvm.org/D83022