RISC-V: Weaken LR/SC pairs

RISC-V: Weaken LR/SC pairs

Introduce the %I and %J flags for setting the .aqrl bits on LR/SC pairs
as needed.

Atomic compare and exchange ops provide success and failure memory
models. C++17 and later place no restrictions on the relative strength
of each model, so ensure we cover both by using a model that enforces
the ordering of both given models.

This change brings LR/SC ops in line with table A.6 of the ISA manual.

2023-04-27 Patrick O'Neill <patrick@rivosinc.com>

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_union_memmodels): Expose
	riscv_union_memmodels function to sync.md.
	* config/riscv/riscv.cc (riscv_union_memmodels): Add function to
	get the union of two memmodels in sync.md.
	(riscv_print_operand): Add %I and %J flags that output the
	optimal LR/SC flag bits for a given memory model.
	* config/riscv/sync.md: Remove static .aqrl bits on LR op/.rl
	bits on SC op and replace with optimized %I, %J flags.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>