Move gmp to here, from the GHC repo

mentioned in commit a2b11abf

mentioned in commit 389dba55

mentioned in commit 9eacabba

mentioned in commit f7ec6f16

mentioned in commit 5b9561d6

mentioned in commit 78f4cca2

mentioned in commit 7c5959a3

mentioned in commit 962571fb

mentioned in commit 6d9c81bd

mentioned in commit f6e629f6

mentioned in commit ce79b3dd

mentioned in commit 19d13d9e

mentioned in commit ghc/ghc@5f4848c0

mentioned in commit a0211d6b

mentioned in commit 06b3efab

mentioned in commit bb440214

mentioned in commit 11bebb6c