Use more efficient uint64x2_t mul code for ARMv7a
Use more efficient uint64x2_t mul code for ARMv7a

Clang 7 and GCC 8 both generate highly inefficient code here for ARMv7a,
repeatedly extracting and reinserting 32-bit registers to do code
that can easily be done entirely in NEON.

Bug reports have been opened in both compilers.

The new code uses fewer instructions, and it is about 2-3 times faster.

I do not know if the code is better or worse on aarch64. I only have
ARMv7 devices at home, so I can't test that. I presume it would be slower,
hence why I disabled it. x86 has a similar effect, doing basically
the same thing. In 32-bit code, it is faster than normal u64 math, but
on 64-bit code, it is faster to leave it alone.

Signed-off-by: easyaspi314 (Devin) <afeb01fd56fe482836773d8d2398f66f37886184@users.noreply.github.com>

xtensor-stack/xsimd,nxblas/nxsimd,jmabille/nxsimd,JohanMabille/xsimd,xtensor-stack/xsimd,JohanMabille/xsimd