misc: add go.mod file

Updates #30228
Updates #30241

Change-Id: I7ee839f4d2840873f7e37b3aff93fe534c6b52e6
Reviewed-on: https://go-review.googlesource.com/c/163207
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

misc/go.mod

+// Module misc contains tests and binaries that pertain to specific build modes
+// (cgo) and platforms (Android and iOS).
+//
+// The 'run' scripts in ../src execute these tests and binaries, which need to
+// be in a module in order to build and run successfully in module mode.
+// (Otherwise, they lack well-defined import paths, and module mode — unlike
+// GOPATH mode — does not synthesize import paths from the absolute working
+// directory.)
+module misc
+
+go 1.12