GMP 6.1.2 t-count_zeros failure on ARM with assertions

[Torbjörn Granlund]

    (Related, I wonder what the effect would be of redefining umul_ppmm as C
    expressions involving __uint128_t on compilers that support that).
    
  We do that already for some CPUs, but this has proven to be somewhat
  fragile, and in unexpected cases lead to libgcc calls.

We brave to do that for at least PowerPC-64, MIPS-64, s390x, Arm64.  For
alpha, gcc provides an _int_mult_upper which we use instead.

Apart from better scheduling, making gcc aware of the semantics allows
for algebraic optimisations and various foldings.

-- 
Torbjörn
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