Stop linking all target libraries in llvm-nm and llvm-objdump.

llvm-nm only needs the target to parse module level assembly in bitcode.  It doesn't need a disassembler or codegen.

llvm-objdump needs to be able to disassemble a file, but doesn't need asm parsers or codegen.

This reduces the sizes of these tools by a few MB each, depending on how many backends are linked in.

llvm-svn: 249632