go.mod: upgrade to go 1.17

This change was produced using 'go mod tidy -go=1.17'
with a go command built at CL 315210.

This activates lazy loading, and updates the go.mod file to maintain
the lazy-loading invariants (namely, including an explicit requirement
for every package transitively imported by the main module).

In particular, this will allow Go 1.17 users who depend on this module
to avoid downloading (or recording checksums for) rsc.io/pdf when
building the packages in their own modules.

Note that this does *not* prevent users with earlier go versions from
successfully building packages from this module.

For golang/go#36460

Change-Id: If7697d9535d8eefd45aa5a42436f3860726f65a2
Reviewed-on: https://go-review.googlesource.com/c/arch/+/315572
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

go.mod

@@ -1,5 +1,5 @@
 module golang.org/x/arch
 
-go 1.11
+go 1.17
 
 require rsc.io/pdf v0.1.1