Resolves #26770

• This patch only fixes compilation with GCC 15 (tested with cross-toolchain from debian experimental)
• I've found that many tests fail on ubuntu 22.04 with qemu before and after this patch. I can't check them on real HW, but the list of failed tests is the same before and after this patch, so we can assume that we haven't break it more than it was. I've created separate issue for this problem: Many test failures on powerpc64le with qemu #26785
• According to Programming reference, on LE platform vec_floate is equivalent to xvcvdpsp and vec_doubleo is equivalent to xvcvspdp. I assume we do not support BE platforms at all.
• I haven't check build with LLVM/clang