Remove hardcoded __arm__ preprocessor definition

I'm not sure why __arm__ must be defined to 1 in this patch, the
compiler should internally define architecture flags such as this.
Also this patch causes big problems when building on x86 Android,
as some libraries (notably Google Breakpad, but probably others)
include boost headers and then erroneously believe that they are
both ARM and x86.
Merge pull request #66 from nikreiman/master

Remove hardcoded __arm__ preprocessor definition