Fix __attribute__((force_align_arg_pointer)) misalignment bug

[Henrik Gramner via cfe-commits]

The force_align_arg_pointer attribute was using a hardcoded 16-byte
alignment value which in combination with -mstack-alignment=32 (or
larger) would produce a misaligned stack which could result in crashes
when accessing stack buffers using aligned AVX load/store
instructions.

The attached patch fixes the issue by using the "stackrealign"
function attribute instead of using a hardcoded 16-byte alignment.

Tested on 64-bit Linux and it works as far as I can see, unable to
test on anything else though.
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