Revert r15: Fix the calling conventions for the sixth argument on i386

The change in r15 was incorrect and it breaks nacl_helper_bootstrap on
Linux, which calls mmap() (which takes 6 arguments) via
linux_syscall_support.h.

BUG=none
TEST=run_hello_world_test in NaCl on x86-32

Review URL: https://codereview.chromium.org/11313020

git-svn-id: http://linux-syscall-support.googlecode.com/svn/trunk/lss@16 829466d3-f3f5-3ae4-62ad-de35cf9bba21