RISC-V: Implement autovec copysign.

RISC-V: Implement autovec copysign.

This adds vector copysign, ncopysign and xorsign as well as the
accompanying tests.

gcc/ChangeLog:

	* config/riscv/autovec.md (copysign<mode>3): Add expander.
	(xorsign<mode>3): Ditto.
	* config/riscv/riscv-vector-builtins-bases.cc (class vfsgnjn):
	New class.
	* config/riscv/vector-iterators.md (copysign): Remove ncopysign.
	(xorsign): Ditto.
	(n): Ditto.
	(x): Ditto.
	* config/riscv/vector.md (@pred_ncopysign<mode>): Split off.
	(@pred_ncopysign<mode>_scalar): Ditto.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/autovec/binop/copysign-run.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/copysign-rv64gcv.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/copysign-rv32gcv.c: New test.
	* gcc.target/riscv/rvv/autovec/binop/copysign-template.h: New test.
	* gcc.target/riscv/rvv/autovec/binop/copysign-zvfh-run.c: New test.