The ARM32 backend used to rely on a specific register declaration order
for calling convention register assignment. This CL introduces a new
field in the ARM32 register tables for explicitly setting which
register holds which parameter.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1508423003 .