Extend RVC to RVC&ZCA.

Extend RVC with ZC extension and compress the instructions, since all ZC extension depend on ZCA, just combine RVC with ZCA.

gcc/ChangeLog:

        * config/riscv/riscv-c.cc (riscv_cpu_cpp_builtins): Mix Target.
        * config/riscv/riscv-shorten-memrefs.cc: Ditto.
        * config/riscv/riscv.cc (riscv_compressed_reg_p): Ditto.
        (riscv_rtx_costs): Ditto.
        (riscv_address_cost): Ditto.
        (riscv_first_stack_step): Ditto.
        * config/riscv/riscv.h (FUNCTION_BOUNDARY): Ditto.