Remove the paragraph about writing to the Free Software Foundation's
mailing address. The FSF has changed addresses in the past, and may do
so again. In 2021 this is not useful, let's rather have a bit less
boilerplate at the start of source files.
Currently we find SSE3 and SSE4.1 code mixed togehter along with
generic code in one file. This introduces the risk that the
compiler exidantly mixes SSE4.1 instructions into an SSE3, or
even worse into a generic code path.
This commit splits the SSE3 and SSE4.1 code into separate files
and compiles them with the matching target options.