This CL will break any uses of 'go tool 5a' etc.
That is intentional.
Code that invokes an assembler directly should be updated to use go tool asm.

We plan to keep the old5a around for bit-for-bit verification during
the release testing phase, but we plan to remove those tools for the
actual release. Renaming the directory now makes sure that lingering
references to 'go tool 5a' will be caught, changed to use asm, and
tested during the release evaluation.

Change-Id: I98748a7ddb34cc7f1b151c2ef421d3656821f5c2
Reviewed-on: https://go-review.googlesource.com/6366Reviewed-by: Rob Pike <r@golang.org>