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

[arlosi]

Summary

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).

[smeenai]

LGTM

llvm/tools/llvm-shlib/CMakeLists.txt

167

I guess this works and is a no-op, but setting this variable to empty (or not setting it at all) should also work, I believe, and perhaps expresses the intent clearer (that there's no target for llvm-nm and you're just supplying a prebuilt one).

[arlosi]

Set llvm_nm_target to empty string as requested by smeenai.

[arlosi]

I do not have commit access. Could someone land this for me?

[smeenai]

Sure, I'll land it for you.