We can scan stack for return address to get vector arguments passed on
stack.

Tested on Linux/x86-64 and Linux/x32.

	* gcc.target/x86_64/abi/test_varargs-m128.c: New file.
	* gcc.target/x86_64/abi/avx/test_varargs-m256.c: Likewise.
	* gcc.target/x86_64/abi/avx512f/test_varargs-m512.c: Likewise.

From-SVN: r271329