cmd/dist will re-exec itself to detect VFP support at run-time.

Fixes #9732, #12548.

Change-Id: I9ad0c5c7fa3e97bd79a32da372e1a962565bb3af
Reviewed-on: https://go-review.googlesource.com/3973Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>