Rollup merge of #36574 - japaric:link-arg, r=alexcrichton

rustc: implement -C link-arg

this flag lets you pass a _single_ argument to the linker but can be
used _repeatedly_. For example, instead of using:

```
rustc -C link-args='-l bar' (..)
```

you could write

```
rustc -C link-arg='-l' -C link-arg='bar' (..)
```

This new flag can be used with RUSTFLAGS where `-C link-args` has
problems with "nested" spaces:

```
RUSTFLAGS='-C link-args="-Tlayout.ld -nostartfiles"'
```

This passes three arguments to rustc: `-C` `link-args="-Tlayout.ld` and
`-nostartfiles"` to `rustc`. That's not what we meant. But this does
what we want:

```
RUSTFLAGS='-C link-arg=-Tlayout.ld -C link-arg=-nostartfiles`
```

cc rust-lang/rfcs#1509

r? @alexcrichton
cc @Zoxc

This needs a test. Any suggestion?

src/test/run-make/link-arg/Makefile

@@ -0,0 +1,5 @@
+-include ../tools.mk
+RUSTC_FLAGS = -C link-arg="-lfoo" -C link-arg="-lbar" -Z print-link-args
+
+all:
+	$(RUSTC) $(RUSTC_FLAGS) empty.rs | grep lfoo | grep lbar

src/test/run-make/link-arg/empty.rs

@@ -0,0 +1,11 @@
+// Copyright 2016 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() { }