RISC-V: Rename constraint c0* to k0*

Rename those constraint since we want define other constraint start with
`c`, those constraints are internal and undocumented, so it's fine to
rename.

gcc/ChangeLog:

	* config/riscv/constraints.md (c01): Rename to...
	(k01): ...this.
	(c02): Rename to...
	(k02): ...this.
	(c03): Rename to...
	(k03): ...this.
	(c04): Rename to...
	(k04): ...this.
	(c08): Rename to...
	(k08): ...this.
	* config/riscv/corev.md (riscv_cv_simd_add_h_si): Update
	constraints.
	(riscv_cv_simd_sub_h_si): Ditto.
	(riscv_cv_simd_cplxmul_i_si): Ditto.
	(riscv_cv_simd_subrotmj_si): Ditto.
	* config/riscv/riscv-v.cc (splat_to_scalar_move_p): Update
	constraints.
	* config/riscv/vector-iterators.md (stride_load_constraint):
	Update constraints.
	(stride_store_constraint): Ditto.