MIPS: The new ABI should be allowed to use r8 and r9 for syscalls.

BUG=linux-syscall-support:7
R=mseaborn@chromium.org, petarj@mips.com

Review URL: https://codereview.chromium.org/389673002

git-svn-id: http://linux-syscall-support.googlecode.com/svn/trunk@31 829466d3-f3f5-3ae4-62ad-de35cf9bba21