test: use architecture-neutral wrapper for calling vprintf

Different architectures use different types for va_list:

x86_64 uses an 1-length array of struct type[0]:

	typedef struct {
		unsigned int gp_offset;
		unsigned int fp_offset;
		void *overflow_arg_area;
		void *reg_save_area;
	} va_list[1];

aarch64 uses a struct type[1]

	typedef struct {
		void *__stack;
		void *__gr_top;
		void *__vr_top;
		int   __gr_offs;
		int   __vr_offs;
	} va_list;

Consequently, C functions which takes a va_list as an argument,
such as vprintf, may pass va_list in different ways depending on
the architecture.

On x86_64, va_list is an array type, so parameter decays to a pointer
and passing the address of the va_list is correct.

On aarch64, the va_list struct is passed by value, but since it is
larger than 16 bytes, the parameter is replaced with a pointer to
caller-allocated memory. Thus, passing the address as an l argument
happens to work.

However, this pattern of passing the address of the va_list to
vprintf doesn't extend to other architectures. On riscv64, va_list
is defined as

	typedef void *va_list;

which is *not* passed by reference. This means that tests that call
vprintf using the address of a va_list (vararg1 and vararg2) will
not work on riscv.

To fix this while keeping the tests architecture-neutral, add a
small wrapper function to the driver which takes a va_list *, and
let the C compiler deal with the details of passing va_list by
value.

[0] https://c9x.me/compile/bib/abi-x64.pdf#figure.3.34
[1] https://c9x.me/compile/bib/abi-arm64.pdf#%5B%7B%22num%22%3A63%2C%22gen%22%3A0%7D%2C%7B%22name%22%3A%22XYZ%22%7D%2C52%2C757%2C0%5D
[2] https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-cc.adoc#va_list-va_start-and-va_arg$

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