support LTO against libraries built with codegen-units > 1
auto merge of #17358 : epdtry/rust/pcg-lto, r=alexcrichton

Building a library with codegen-units > 1 will place several `crate.0.bytecode.deflate` files in the rlib.  Linking with LTO loads all the `crate.0.bytecode.deflate` files it can find.

r? @alexcrichton