Use sha256 for hashes in the release process

I just came across the GDB 10.1 release notes and saw that md5 is still
being used in those. I thought it would be a good idea to instead have a
more modern, secure and wildly available hash function such as SHA256 as
part of the release process.

The changes have been done rather mechnically via sed but executing the
`src-release.sh -b gdb` did work so I am confident about the result.

While this does not directly address the release mails, I was wasn't
able to find the template/script used for those, this is probably still
an improvement.

ChangeLog:
	* src-release.sh: Use sha256sum instead of md5sum.

binutils/ChangeLog:
	* README-how-to-make-a-release: Use sha256sum instead of md5sum.

Change-Id: I9cf19ea40699137c45463b8514f6e29271af2347