rust-llvm: Compile llvm to use dynamic libraries

rust-llvm: Compile llvm to use dynamic libraries

Our main llvm recipe uses dynamic linking already but rust-llvm does not.
Enabling this significanly reduces the size of llvm-rust to about a third
of what it was, which is a bettter configuration for us leading to smaller
TMPDIR and sstate objects.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>