Detect NaCl osabi from objfile

This is needed to obtain correct gdbarch structure when doing get_objfile_arch.

In particular, with correct gdbarch structure GDB calls correct gdbarch_skip_prologue hook when setting breakpoints on function start.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2898

Review URL: https://chromiumcodereview.appspot.com/10829380