Since b567b38c (target-arm: remove T0 and T1, 2009-10-16) the only global
register that is used is AREG0, so the complexity of hostregs_helper.h
is unused.  Use regular assignments and a compiler optimization barrier.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>