README: Delete obsolete i386 info + update arch/i386/ paths

Support for i386 was removed in v3.8, delete the paragraph that says
processor types above 386 won't work on that architecture. It's obsolete
information and potentially confusing. Also change a couple of
"arch/i386/" paths to one that exists now, using "arch/x86/" instead.

Signed-off-by: Øyvind A. Holm <sunny@sunbase.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>