MIPS: tweak register constraint to build under clang

MIPS: tweak register constraint to build under clang

The constraints used for passing in the syscall number breaks when
building with clang:
.../lss/linux_syscall_support.h:2977:14: error: invalid input constraint '0' in asm
  LSS_INLINE _syscall1(void *,  brk,             void *,      e)
             ^

This might be a bug in clang itself, but we can tweak the constraints
on our side so it works with gcc & clang.

BUG=http://llvm.org/bugs/show_bug.cgi?id=21613
BUG=linux-syscall-support:9
TEST=gcc generates same asm before & after
TEST=clang now compiles
R=benchan@chromium.org

Committed: https://code.google.com/p/linux-syscall-support/source/detail?r=33

[vapier, 2015-01-16 12:06:09]

vapier@chromium.org changed reviewers:
+ benchan@chromium.org, thestig@chromium.org

[Ben Chan, 2015-01-22 16:44:33]

lgtm

[vapier, 2015-01-22 16:46:43]

Committed patchset #1 (id:1) manually as r33 (presubmit successful).