With multiblock and -n4000, at least one block in geekbench5's camera
benchmark was spilling 324 values. This meant the required stack
adjustment was 5184 bytes, which is larger than can fit into a 12bit
immediate.
Now, it's probally a bug that our RA is spilling that many values, and
we probally should be packing our spill slots closer together, but we
still shouldn't fail to run in this edge case.
So this commit adds a fallback path which uses a temp register to load
the stack adjustment.
Scott Mansell
c4dda4a3f3