This CL passes in context from the ARM target lowering, so that
it can figure out what register (SP or FP) and offset to use
when loading/storing variables.

BUG= https://bugs.chromium.org/p/nativeclient/issues/detail?id=4334
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1414043015 .