Add a stub __cxa_demangle to disable LLVM's demangler.

Add a stub __cxa_demangle to disable LLVM's demangler.

LLVM's demangler is fairly large and included by default in linked
binaries.  Chromium's demangling happens on host systems when needed,
so demangling code is not required on the target.

Supplying our own stub function stops the linker pulling in the LLVM
demangler from libc++, and reduces the size of libchrome.so accordingly.

Results from a Release build:
  Before:
    $ size --format=SysV out/Release/chrome_apk/libs/armeabi-v7a/libchrome.so
    section                      size       addr
    ...
    .text                    26210064    2237952
    ...
    Total                    35736055
  After:
    $ size --format=SysV out/Release/chrome_apk/libs/armeabi-v7a/libchrome.so
    section                      size       addr
    ...
    .text                    26007820    2237888
    ...
    Total                    35523543

BUG=485154

Review URL: https://codereview.chromium.org/1215673002

Cr-Commit-Position: refs/heads/master@{#336765}