[libc] Add more test cases to the argument list tests

This patch adds some extra cases to the existing argument list test in
`libc`, mainly dealing with arguments of varying sizes and primitive
types.

The purpose of this patch is to provide a wider test area when we begin
to provide varargs support on the GPU as there is no other runtime code
that really tests it. So, running these tests more exhaustively in the
GPU libc project will serve as the runtime tests for GPU vararg support in
D158246.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D158867