Disable generation of scalar modulo instructions.

Disable generation of scalar modulo instructions.

It was recently discovered that the scalar modulo instructions can suffer
noticeable performance issues for certain input values. This patch disables
their generation since the equivalent div/mul/sub sequence does not suffer
the same problem.

gcc/
	* config/rs6000/rs6000.cc (rs6000_rtx_costs): Check whether the
	modulo instruction is disabled.
	* config/rs6000/rs6000.h (RS6000_DISABLE_SCALAR_MODULO): New.
	* config/rs6000/rs6000.md (mod<mode>3, *mod<mode>3): Check it.
	(define_expand umod<mode>3): New.
	(define_insn umod<mode>3): Rename to *umod<mode>3 and check if the modulo
	instruction is disabled.
	(umodti3, modti3): Check if the modulo instruction is disabled.

gcc/testsuite/
	* gcc.target/powerpc/clone1.c: Add xfails.
	* gcc.target/powerpc/clone3.c: Likewise.
	* gcc.target/powerpc/mod-1.c: Update scan strings and add xfails.
	* gcc.target/powerpc/mod-2.c: Likewise.
	* gcc.target/powerpc/p10-vdivq-vmodq.c: Add xfails.