mk: tell rustc that we're only looking for native libs in the LLVM_LIBDIR
mk: tell rustc that we're only looking for native libs in the LLVM_LIBDIR

This fixes the case where we try to re-build & re-install rust to the
same prefix (without uninstalling) while using an llvm-root that is the
same as the prefix.

Without this, builds like that fail with:
	'error: multiple dylib candidates for `std` found'

See https://github.com/jmesmon/meta-rust/issues/6 for some details.

May also be related to #20342.

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust