RISC-V: Minimal support ZC extensions.

This patch is the minimal support for ZC* extensions, include the extension
name, mask and target defination. Also define the dependencies with Zca
and Zce extension. Notes that all ZC* extensions depend on the Zca extension.
Zce includes all relevant ZC* extensions for microcontrollers using.

Co-Authored by: Charlie Keaney <charlie.keaney@embecosm.com>
Co-Authored by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>
Co-Authored by: Simon Cook <simon.cook@embecosm.com>

gcc/ChangeLog:

        * common/config/riscv/riscv-common.cc: New extensions.
        * config/riscv/riscv-opts.h (MASK_ZCA): New mask.
        (MASK_ZCB): Ditto.
        (MASK_ZCE): Ditto.
        (MASK_ZCF): Ditto.
        (MASK_ZCD): Ditto.
        (MASK_ZCMP): Ditto.
        (MASK_ZCMT): Ditto.
        (TARGET_ZCA): New target.
        (TARGET_ZCB): Ditto.
        (TARGET_ZCE): Ditto.
        (TARGET_ZCF): Ditto.
        (TARGET_ZCD): Ditto.
        (TARGET_ZCMP): Ditto.
        (TARGET_ZCMT): Ditto.
        * config/riscv/riscv.opt: New target variable.