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>

Makefile.am

@@ -593,6 +593,10 @@ src_tools_linux_dump_syms_dump_syms_SOURCES = \
 	src/common/linux/memory_mapped_file.cc \
 	src/common/linux/safe_readlink.cc \
 	src/tools/linux/dump_syms/dump_syms.cc
+src_tools_linux_dump_syms_dump_syms_CXXFLAGS = \
+	$(RUST_DEMANGLE_CFLAGS)
+src_tools_linux_dump_syms_dump_syms_LDADD = \
+	$(RUST_DEMANGLE_LIBS)
 
 src_tools_linux_md2core_minidump_2_core_SOURCES = \
 	src/common/linux/memory_mapped_file.cc \
@@ -641,7 +645,10 @@ src_tools_mac_dump_syms_dump_syms_mac_SOURCES = \
 	src/tools/mac/dump_syms/dump_syms_tool.cc
 src_tools_mac_dump_syms_dump_syms_mac_CXXFLAGS= \
 	-I$(top_srcdir)/src/third_party/mac_headers \
+	$(RUST_DEMANGLE_CFLAGS) \
 	-DHAVE_MACH_O_NLIST_H
+src_tools_mac_dump_syms_dump_syms_mac_LDADD= \
+	$(RUST_DEMANGLE_LIBS)
 
 src_common_dumper_unittest_SOURCES = \
 	src/common/byte_cursor_unittest.cc \
@@ -698,9 +705,11 @@ src_common_dumper_unittest_SOURCES = \
 	src/common/tests/file_utils.cc
 src_common_dumper_unittest_CPPFLAGS = \
 	$(AM_CPPFLAGS) $(TEST_CFLAGS) \
+	$(RUST_DEMANGLE_CFLAGS) \
 	$(PTHREAD_CFLAGS)
 src_common_dumper_unittest_LDADD = \
 	$(TEST_LIBS) \
+	$(RUST_DEMANGLE_LIBS) \
 	$(PTHREAD_CFLAGS) $(PTHREAD_LIBS)
 
 src_common_mac_macho_reader_unittest_SOURCES = \