Move add_rlib and add_native_library to cg_ssa
Move add_rlib and add_native_library to cg_ssa

This deduplicates logic between codegen backends

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