C-SKY: Support fldrd/fstrd for fpuv2 and fldr.64/fstr.64 for fpuv3.

C-SKY: Support fldrd/fstrd for fpuv2 and fldr.64/fstr.64 for fpuv3.

gcc/ChangeLog:

	* config/csky/csky.c (ck810_legitimate_index_p): Support
	"base + index" with DF mode.
	* config/csky/constraints.md ("Y"): New constraint for memory operands
	without index register.
	* config/csky/csky_insn_fpuv2.md (fpuv3_movdf): Use "Y" instead of "m"
	when mov between memory and general registers, and lower their priority.
	* config/csky/csky_insn_fpuv3.md (fpuv2_movdf): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.target/csky/fldrd_fstrd.c: New.
	* gcc.target/csky/fpuv3/fldr64_fstr64.c: New.