rs6000: Fix allocate_stack in a corner case (PR91289)

rs6000: Fix allocate_stack in a corner case (PR91289)

When we have -fstack-limit-symbol with sysv we can end up with a non-
existing instruction (you cannot add an immediate to register 0).  Fix
this by using register 11 instead.  It might be used for something else
already though, so save and restore its value around this.  In
optimizing compiles these extra moves are usually removed again: the
restore by cprop_hardreg, and then the save by rtl_dce.

	PR target/91289
	* config/rs6000/rs6000-logue.c (rs6000_emit_allocate_stack): Don't add
	an immediate to r0; use r11 instead.  Save and restore r11 to r0 around
	this.

From-SVN: r277472