This:

1) Defines the ABI hash of a package (as the SHA1 of the __.PKGDEF)
2) Defines the ABI hash of a shared library (sort the packages by import
   path, concatenate the hashes of the packages and SHA1 that)
3) When building a shared library, compute the above value and define a
   global symbol that points to a go string that has the hash as its value.
4) When linking against a shared library, read the abi hash from the
   library and put both the value seen at link time and a reference
   to the global symbol into the moduledata.
5) During runtime initialization, check that the hash seen at link time
   still matches the hash the global symbol points to.

Change-Id: Iaa54c783790e6dde3057a2feadc35473d49614a5
Reviewed-on: https://go-review.googlesource.com/8773Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>