Rollup merge of #47453 - pftbest:nointas, r=alexcrichton

Fix no_integrated_as option to work with new codegen architecture.

Old implementation called the assembler once per crate, but we need to call
it for each object file instead, because a single crate can now have more
than one object file.

This patch fixes issue #45836 (Can't compile core for msp430 in release mode)

This change can be tested on x86_64 using
```sh
export RUSTFLAGS="-C no_integrated_as -C save_temps"
```

r? @alexcrichton
cc @japaric

src/test/run-make/no-integrated-as/Makefile

@@ -0,0 +1,7 @@
+-include ../tools.mk
+
+all:
+ifeq ($(TARGET),x86_64-unknown-linux-gnu)
+	$(RUSTC) hello.rs -C no_integrated_as
+	$(call RUN,hello)
+endif

src/test/run-make/no-integrated-as/hello.rs

@@ -0,0 +1,13 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    println!("Hello, world!");
+}