-Z linker-flavor

-Z linker-flavor

This patch adds a `-Z linker-flavor` flag to rustc which can be used to invoke
the linker using a different interface.

For example, by default rustc assumes that all the Linux targets will be linked
using GCC. This makes it impossible to use LLD as a linker using just `-C
linker=ld.lld` because that will invoke LLD with invalid command line
arguments. (e.g. rustc will pass -Wl,--gc-sections to LLD but LLD doesn't
understand that; --gc-sections would be the right argument)

With this patch one can pass `-Z linker-flavor=ld` to rustc to invoke the linker
using a LD-like interface. This way, `rustc -C linker=ld.lld -Z
linker-flavor=ld` will invoke LLD with the right arguments.

`-Z linker-flavor` accepts 4 different arguments: `em` (emcc), `ld`,
`gcc`, `msvc` (link.exe). `em`, `gnu` and `msvc` cover all the existing linker
interfaces. `ld` is a new flavor for interfacing GNU's ld and LLD.

This patch also changes target specifications. `linker-flavor` is now a
mandatory field that specifies the *default* linker flavor that the target will
use. This change also makes the linker interface *explicit*; before, it used to
be derived from other fields like linker-is-gnu, is-like-msvc,
is-like-emscripten, etc.

Another change to target specifications is that the fields `pre-link-args`,
`post-link-args` and `late-link-args` now expect a map from flavor to linker
arguments.

``` diff
-    "pre-link-args": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+    "pre-link-args": {
+        "gcc": ["-Wl,--as-needed", "-Wl,-z,-noexecstack"],
+        "ld": ["--as-needed", "-z,-noexecstack"],
+    },
```

[breaking-change]  for users of custom targets specifications