RISC-V: Minimal support for 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. Zce
will imply zcf when 'f' extension enabled in rv32.

Co-Authored by: Charlie Keaney <charlie.keaney@embecosm.com>
Co-Authored by: Mary Bennett <mary.bennett@embecosm.com>
Co-Authored by: Nandni Jamnadas <nandni.jamnadas@embecosm.com>
Co-Authored by: Simon Cook <simon.cook@embecosm.com>
Co-Authored by: Sinan Lin <sinan.lin@linux.alibaba.com>
Co-Authored by: Shihua Liao <shihua@iscas.ac.cn>
Co-Authored by: Yulong Shi <yulong@iscas.ac.cn>

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_subset_list::parse): 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.