[llvm-shlib] Fix cross-compilation for LLVM-C

When we're cross-compiling, build and use a native llvm-nm instead of
attempting to use the one from the target's build tree.

A nice follow-up would be to add a cache variable to allow specifying a
path to an external native llvm-nm instead of building one ourselves,
similar to LLVM_TABLEGEN and LLVM_CONFIG_PATH.

Differential Revision: https://reviews.llvm.org/D60025

llvm-svn: 357487