Permit ABIs where the caller extends the result (test change).

This test was written to make sure *something* sane is generated, not
to test any ABI's signedness semantics.

This should allow the test to pass if AArch64 is the default target.
XFAIL test that's inappropriate for AArch64 ABI

Only some ABIs require the "signext" attribute on parameters. On most
platforms, however, it's a useful test so it's best not to limit it to some
random arbitrary platform.
Add AArch64 wchar definition to test

This should allow it to pass if the default triple is AArch64
Only check for volatile memcpys in test.

AArch64 handles aggFct's return struct slightly differently, leading
to an extra memcpy. This test is fortunately only concerned about
volatile copies, so we can modify the grep text to filter it.