Update gmp to 6.0.0a and enable building on both darwin-32 and darwin-64
Update gmp to 6.0.0a and enable building on both darwin-32 and darwin-64

Update gmp to 6.0.0a and enable building on both darwin-32 and darwin-64

# Upstream changelog
https://gmplib.org/gmp6.0.html

Bug fixes

 * The function mpz_invert now considers any number invertible in Z/1Z.
 * The mpn multiply code now handles operands of more than 231 limbs
correctly. (Note however that the mpz code is limited to 232 bits on
32-bit hosts and 237 bits on 64-bit hosts.)

Performance enhancements

 * Plain division of large operands is faster and more monotonous in
operand size.
 * Major speedup for ARM, in particular ARM Cortex-A15, thanks to
improved assembly.
 * Major speedup for SPARC T4/T5 and speedup also for T3, thanks to a
lot of new assembly.
 * Speedup for Intel Sandy Bridge, Ivy Bridge, Haswell, thanks to
rewritten and vastly expanded assembly support. Speedup also for the
older Core 2 and Nehalem.
 * Faster mixed arithmetic between mpq_class and double.
 * With g++, optimise more operations when one argument is a simple
constant.

Features

 * Support for new Intel and AMD CPUs.
 * Support for ARM64 alias Aarch64 alias ARMv8.
 * New public functions mpn_sec_mul and mpn_sec_sqr, implementing
side-channel silent multiplication and squaring.
 * New public functions mpn_sec_div_qr and mpn_sec_div_r, implementing
side-channel silent division.
 * New public functions mpn_cnd_add_n and mpn_cnd_sub_n. Side-channel
silent conditional addition and subtraction.
 * New public function mpn_sec_powm, implementing side-channel silent
modexp.
 * New public function mpn_sec_invert, implementing side-channel silent
modular inversion.
 * Better support for applications which use the mpz_t type, but
nevertheless need to call some of the lower-level mpn functions. See
the documentation for mpz_limbs_read and related functions.

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