Fix testsuite for C23 `va_start` (#861)

Fix testsuite for C23 `va_start` (#861)

In the C23 revision of the C standard, `va_start` ignores its second
argument, which is no longer required (previously the last named
function parameter - which the compiler knows anyway, so it's
redundant information).

This has the consequence for the libffi testsuite, when making GCC
default to `-std=gnu23`, of making two tests fail with warnings about
an unused function argument (only passed to `va_start` and not
otherwise used).  Fix those test failures by explicitly casting the
argument to `void`.