RISC-V: Implement TARGET_MANGLE_DECL_ASSEMBLER_NAME

This patch implements the TARGET_MANGLE_DECL_ASSEMBLER_NAME for RISC-V.
This is used to add function multiversioning suffixes to the assembler
name.

Signed-off-by: Yangyu Chen <cyy@cyyself.name>

gcc/ChangeLog:

	* config/riscv/riscv.cc
	(riscv_mangle_decl_assembler_name): New function.
	(TARGET_MANGLE_DECL_ASSEMBLER_NAME): Define.