s390: Add -munroll-only-small-loops.

s390: Add -munroll-only-small-loops.

Inspired by Power we also introduce -munroll-only-small-loops.  This
implies activating -funroll-loops and -munroll-only-small-loops at -O2 and
above.

gcc/ChangeLog:

	* common/config/s390/s390-common.cc: Enable -funroll-loops and
	-munroll-only-small-loops for OPT_LEVELS_2_PLUS_SPEED_ONLY.
	* config/s390/s390.cc (s390_loop_unroll_adjust): Do not unroll
	loops larger than 12 instructions.
	(s390_override_options_after_change): Set unroll options.
	(s390_option_override_internal): Likewise.
	* config/s390/s390.opt: Document munroll-only-small-loops.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/vector/vec-copysign.c: Do not unroll.
	* gcc.target/s390/zvector/autovec-double-quiet-uneq.c: Dito.
	* gcc.target/s390/zvector/autovec-double-signaling-ltgt.c: Dito.
	* gcc.target/s390/zvector/autovec-float-quiet-uneq.c: Dito.
	* gcc.target/s390/zvector/autovec-float-signaling-ltgt.c: Dito.