Handle the sse4-x86 test correctly when Valgrind compiled with Clang

Handle the sse4-x86 test correctly when Valgrind compiled with Clang

The none/tests/x86/sse4-x86 fails when valgrind is compiled with clang.
With default clang -O2 (which involves the slp-vectorizer optimization)
this test code translates to assembly involving MOVDQA.  That's a SIMD
instruction requiring explicitly aligned memory.

Turns out that the required 16-byte stack alignment isn't in place by
default.  This instruction ends up with a GP, so that Valgrind ends up
with segmentation fault.

To fix this, the affected clean helper function g_calc_mpsadbw() is
now compiled with the 'force_align_arg_pointer' attribute, making sure
the needed stack alignment is in place.  Both GCC and clang accept
this attribute.

https://bugs.kde.org/show_bug.cgi?id=523626