Rename SCRATCH to RSCRATCH. Never stack RSCRATCH nor FSCRATCH.

Rename SCRATCH to RSCRATCH.  Never stack RSCRATCH nor FSCRATCH.

Rename the scratch gpr (currently r11) from SCRATCH to RSCRATCH so I
can search for RSCRATCH without finding FSCRATCH.  I also want to
avoid confusion with the SCRATCH keyword of the old code generator (cg
which came before ncg).

Change the stacking rules to prevent stacking of RSCRATCH or FSCRATCH
or any other GPR or FPR that isn't an allocatable REG or FREG.  Then
ncgg rejects any rule that tries to stack a GPR or FPR, so change such
rules to stack a REG or FREG.