rust: Set `-C metadata` on proc-macros

Without this set, trying to load two proc-macros with the same crate
name but different hashes may fail quietly at the interner step.

Bug: 383526333
Test: m crosvm using both zerocopy-0.8 and zerocopy-0.7
Change-Id: I32b70fd8eb278c3edb649f268f762761082a46b7