Add -C link-dead-code option r=alexcrichton

Turning gc-sections off improves code coverage based for tools which
use DWARF debugging information (like kcov). Otherwise dead code is
stripped and kcov returns a coverage percentage that doesn't reflect
reality.

src/librustc/session/config.rs

@@ -507,6 +507,8 @@ options! {CodegenOptions, CodegenSetter, basic_codegen_options,
         "system linker to link outputs with"),
     link_args: Option<Vec<String>> = (None, parse_opt_list,
         "extra arguments to pass to the linker (space separated)"),
+    link_dead_code: bool = (false, parse_bool,
+        "let the linker strip dead coded (turning it on can be used for code coverage)"),
     lto: bool = (false, parse_bool,
         "perform LLVM link-time optimizations"),
     target_cpu: Option<String> = (None, parse_opt_string,