[committed] [RISC-V] Allow uarchs to set TARGET_OVERLAP_OP_BY_PIECES_P

[committed] [RISC-V] Allow uarchs to set TARGET_OVERLAP_OP_BY_PIECES_P

This is almost exclusively work from the VRULL team.

As we've discussed in the Tuesday meeting in the past, we'd like to have a knob
in the tuning structure to indicate that overlapped stores during
move_by_pieces expansion of memcpy & friends are acceptable.

This patch adds the that capability in our tuning structure.  It's off for all
the uarchs upstream, but we have been using it inside Ventana for our uarch
with success.  So technically it's NFC upstream, but puts in the infrastructure
multiple organizations likely need.

gcc/

	* config/riscv/riscv.cc (struct riscv_tune_param): Add new
	"overlap_op_by_pieces" field.
	(rocket_tune_info, sifive_7_tune_info): Set it.
	(sifive_p400_tune_info, sifive_p600_tune_info): Likewise.
	(thead_c906_tune_info, xiangshan_nanhu_tune_info): Likewise.
	(generic_ooo_tune_info, optimize_size_tune_info): Likewise.
	(riscv_overlap_op_by_pieces): New function.
	(TARGET_OVERLAP_OP_BY_PIECES_P): define.

gcc/testsuite/

	* gcc.target/riscv/memcpy-nonoverlapping.c: New test.
	* gcc.target/riscv/memset-nonoverlapping.c: New test.