codegen_ssa: multiply scalable vec size by vscale

When emitting a `memcpy` for a scalable vector, the size computed by
rustc (`num_vectors * element_count * element_ty`), since
rust-lang/rust#157915, needs to be multiplied by `vscale`.

Author: davidtwco

src/builder.rs

@@ -1459,6 +1459,10 @@ impl<'a, 'gcc, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'gcc, 'tcx> {
         );
     }
 
+    fn vscale(&mut self, _: Self::Type) -> Self::Value {
+        unimplemented!("`rustc_codegen_gcc` doesn't support scalable vectors yet")
+    }
+
     fn select(
         &mut self,
         cond: RValue<'gcc>,