cmd: don't require build tags in relnote pathnames

cmd: don't require build tags in relnote pathnames

Due to a bug in golang.org/x/build/relnote, API features affecting
specific builds would need to include those build tags in relnote
pathnames.

This CL vendors in the fixed golang.org/x/build. (That caused other
modules to be vendored in as well.)

It also renames the syscall relnote file to remove the build tags
from its pathname.

For #64169.

Change-Id: Iaf6cd9099df1156f4e20c63d519a862ea19a7a3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/566455
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>