RISC-V: Eliminate SYNC memory models

Remove references to MEMMODEL_SYNC_* models by converting via
memmodel_base().

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

gcc/ChangeLog:

	* config/riscv/riscv.cc: Remove MEMMODEL_SYNC_* cases and
	sanitize memmodel input with memmodel_base.

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