RISC-V: Part-3: Output .variant_cc directive for vector function

RISC-V: Part-3: Output .variant_cc directive for vector function

Functions which follow vector calling convention variant need be annotated by
.variant_cc directive according the RISC-V Assembly Programmer's Manual[1] and
RISC-V ELF Specification[2].

[1] https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md#pseudo-ops
[2] https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/master/riscv-elf.adoc#dynamic-linking

gcc/ChangeLog:

	* config/riscv/riscv-protos.h (riscv_declare_function_name): Add protos.
	(riscv_asm_output_alias): Ditto.
	(riscv_asm_output_external): Ditto.
	* config/riscv/riscv.cc (riscv_asm_output_variant_cc):
	Output .variant_cc directive for vector function.
	(riscv_declare_function_name): Ditto.
	(riscv_asm_output_alias): Ditto.
	(riscv_asm_output_external): Ditto.
	* config/riscv/riscv.h (ASM_DECLARE_FUNCTION_NAME):
	Implement ASM_DECLARE_FUNCTION_NAME.
	(ASM_OUTPUT_DEF_FROM_DECLS): Implement ASM_OUTPUT_DEF_FROM_DECLS.
	(ASM_OUTPUT_EXTERNAL): Implement ASM_OUTPUT_EXTERNAL.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/rvv/base/abi-call-variant_cc.c: New test.