checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/libatomic-ops/package.py

@@ -14,7 +14,7 @@ class LibatomicOps(AutotoolsPackage):
     url      = "https://www.hboehm.info/gc/gc_source/libatomic_ops-7.6.6.tar.gz"
 
     version('7.6.6', sha256='99feabc5f54877f314db4fadeb109f0b3e1d1a54afb6b4b3dfba1e707e38e074')
-    version('7.4.4', '426d804baae12c372967a6d183e25af2')
+    version('7.4.4', sha256='bf210a600dd1becbf7936dd2914cf5f5d3356046904848dcfd27d0c8b12b6f8f')
 
     def configure_args(self):
         return ['--enable-shared']