Changes to support cross-compiling LLVM. The GenLibDeps.pl script needs to

have a compile-host version of "nm", not build-host. In order to effect this
we must use autoconf to determine the correct "nm" to use and propagate that
through the makefiles, through llvm-config and finally to GenLibDeps.pl as
an optional argument.
Patch contributed by Anton Korobeynikov. Thanks!

llvm-svn: 29368