Use ARM-style representation for C++ method pointers under PNaCl/Emscripten

Before this change, Clang uses the x86 representation for C++ method
pointers when generating code for PNaCl.  However, the resulting code
will assume that function pointers are 0 mod 2.  This assumption is
not safe for PNaCl, where function pointers could have any value
(especially in future sandboxing models).

So, switch to using the ARM representation for PNaCl code, which makes
no assumptions about the alignment of function pointers.

Since we're changing the "le32" target, this change also applies to
Emscripten.  The change is beneficial for Emscripten too.  Emscripten
has a workaround to make function pointers 0 mod 2.  This change would
allow the workaround to be removed.

See: https://code.google.com/p/nativeclient/issues/detail?id=3450