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/dwarf/dwarf2enums.h

@@ -544,6 +544,7 @@ enum DwarfLanguage
     DW_LANG_ObjC_plus_plus           =0x0011,
     DW_LANG_UPC                      =0x0012,
     DW_LANG_D                        =0x0013,
+    DW_LANG_Rust                     =0x001c,
     DW_LANG_Swift                    =0x001e,
     // Implementation-defined language code range.
     DW_LANG_lo_user = 0x8000,