libffi: fix handling of homogeneous float128 structs (#689)

If there is a homogeneous struct with float128 members, they should be
copied to vector register save area. The current code incorrectly copies
only the value of the first member, not increasing the pointer with each
iteration. Fix this.

Merged from upstream libffi commit: 464b4b66e3cf3b5489e730c1466ee1bf825560e0

2023-05-03  Dan Horák <dan@danny.cz>

libffi/
	PR libffi/109447
	* src/powerpc/ffi_linux64.c (ffi_prep_args64): Update arg.f128 pointer.

(cherry picked from commit 043550bc)