RISC-V: Add Multi-Versioning Test Cases

This patch adds test cases for the Function Multi-Versioning (FMV)
feature for RISC-V, which reuses the existing test cases from the
aarch64 and ported them to RISC-V.

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

gcc/testsuite/ChangeLog:

	* g++.target/riscv/mv-symbols1.C: New test.
	* g++.target/riscv/mv-symbols2.C: New test.
	* g++.target/riscv/mv-symbols3.C: New test.
	* g++.target/riscv/mv-symbols4.C: New test.
	* g++.target/riscv/mv-symbols5.C: New test.
	* g++.target/riscv/mvc-symbols1.C: New test.
	* g++.target/riscv/mvc-symbols2.C: New test.
	* g++.target/riscv/mvc-symbols3.C: New test.
	* g++.target/riscv/mvc-symbols4.C: New test.