RISC-V: Minimal support of z*inx extension.

Minimal support of z*inx extension, include 'zfinx'， 'zdinx' and 'zhinx/zhinxmin'
corresponding to 'f', 'd' and 'zfh/zfhmin', the 'zdinx' will imply 'zfinx'
same as 'd' imply 'f', 'zhinx' will aslo imply 'zfinx', all zfinx extension imply 'zicsr'.

Co-Authored-By: Sinan Lin <sinan@isrc.iscas.ac.cn>

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc: New extensions.
	* config/riscv/arch-canonicalize: New imply relations.
	* config/riscv/riscv-opts.h (MASK_ZFINX): New mask.
	(MASK_ZDINX): Ditto.
	(MASK_ZHINX): Ditto.
	(MASK_ZHINXMIN): Ditto.
	(TARGET_ZFINX): New target.
	(TARGET_ZDINX): Ditto.
	(TARGET_ZHINX): Ditto.
	(TARGET_ZHINXMIN): Ditto.
	* config/riscv/riscv.opt: New target variable.