initial import of GMP 6.2.1. from their NEWS:

Changes between GMP version 6.2.0 and 6.2.1

BUGS FIXED
* A possible overflow of type int is avoided for mpz_cmp on huge operands.
* Overflows are more carefully detected and reported for mpz_pow_ui.
* A bug in longlong.h for aarch64 sub_ddmmss, not affecting GMP, was healed.

FEATURES
* C90 compliance.
* Initial support for Darwin on arm64, and improved portability.
* Support for more processors.

initial import of GMP 6.2.0. changes include:

- Bug fixes to gmp_snprintf, conversion to double, mpz_powm,
and mpf_set_str.
- New functions for factorial, primorial, fibonacci, mpz_2fac_ui,
and mpz_mfac_uiui.
- MIPS r6 cores are now supported.
- Various speeds ups.