Bignum: implement extended GCD (#18427)

libraries/ghc-bignum/cbits/gmp_wrappers.c

libraries/ghc-bignum/src/GHC/Num/Backend/Check.hs

libraries/ghc-bignum/src/GHC/Num/Backend/FFI.hs

libraries/ghc-bignum/src/GHC/Num/Backend/GMP.hs

libraries/ghc-bignum/src/GHC/Num/Backend/Native.hs

libraries/ghc-bignum/src/GHC/Num/BigNat.hs-boot

libraries/ghc-bignum/src/GHC/Num/Integer.hs

libraries/ghc-bignum/src/GHC/Num/Integer.hs-boot

libraries/integer-gmp/src/GHC/Integer/GMP/Internals.hs

testsuite/tests/lib/integer/all.T

testsuite/tests/lib/integer/gcdeInteger.hs

testsuite/tests/lib/integer/gcdeInteger.stdout

testsuite/tests/lib/integer/integerGcdExt.hs