RISC-V: Strengthen atomic stores

RISC-V: Strengthen atomic stores

This change makes atomic stores strictly stronger than table A.6 of the
ISA manual. This mapping makes the overall patchset compatible with
table A.7 as well.

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

	PR target/89835

gcc/ChangeLog:

	* config/riscv/sync.md (atomic_store<mode>): Use simple store
	instruction in combination with fence(s).

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/pr89835.c: New test.

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