target-mips: Pass env to atomic load/store helpers.

IMPORTANT NOTE: This contains a gross hack to route around a very
nasty GCC 4.6 compiler bug (on Linux), that prevented helper_ll()
from being compiled correctly, resulting in failure to boot a
valid MIPS system image!!

Change-Id: I20f084e3966a7160cd283f2aa4d391799abc667c
3 files changed