Use [profile.release.build-override] to optimize build dependencies. (#437)

Author: eddyb

Cargo.toml

@@ -12,3 +12,9 @@ members = [
     "crates/spirv-builder",
     "crates/spirv-std",
 ]
+
+# Compile build-dependencies in release mode with
+# the same settings as regular dependencies.
+[profile.release.build-override]
+opt-level = 3
+codegen-units = 16