Disable use of R4 as a promotable register

When we are using implicit suspend checks we can potentially
use r4 as a register into which variables can be promoted.  However
the runtime doesn't save this and thus will corrupt it.  Not good.

This disables the promotion of r4 until we can figure out how
to make the runtime save it properly.

Change-Id: Ib95ce93579e1c364de5ecc8e728f2cb7990da77a