Don't demangle Rust symbols by default, but allow linking to rust-demangle.

The Rust compiler uses GCC C++ name mangling, but it has another layer of
encoding so abi::cxa_demangle doesn't produce great results. This patch
changes dump_syms to dump unmangled names by default so that consumers can
demangle them after-the-fact.

It also adds a tiny bit of support for linking against a Rust library I wrote
that can demangle Rust symbols nicely:
https://github.com/luser/rust-demangle-capi

BUG=

Change-Id: I63a425035ebb7ac516f067fed2aa782849ea9604
Reviewed-on: https://chromium-review.googlesource.com/402308
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/language.h

@@ -96,6 +96,7 @@ class Language {
   static const Language * const CPlusPlus,
                         * const Java,
                         * const Swift,
+                        * const Rust,
                         * const Assembler;
 };