all: update build constraints to Go 1.17

Do it all at once in preparation for Go 1.18 support.

To make this commit, I've simply modified the `fmt-check` Makefile
target to rewrite files instead of listing the differences. So this is a
fully mechanical change, it should not have introduced any errors.

Author: aykevl

builder/tools-builtin.go

@@ -1,3 +1,4 @@
+//go:build byollvm
 // +build byollvm
 
 package builder

builder/tools-external.go

@@ -1,3 +1,4 @@
+//go:build !byollvm
 // +build !byollvm
 
 package builder

src/device/arm/scb.go

@@ -1,6 +1,7 @@
 // Hand created file. DO NOT DELETE.
 // Cortex-M System Control Block-related definitions.
 
+//go:build cortexm
 // +build cortexm
 
 package arm

src/examples/can/feather-m4-can.go

@@ -1,3 +1,4 @@
+//go:build feather_m4_can
 // +build feather_m4_can
 
 package main

src/examples/caninterrupt/feather-m4-can.go

@@ -1,3 +1,4 @@
+//go:build feather_m4_can
 // +build feather_m4_can
 
 package main

src/examples/dac/circuitplay_express.go

@@ -1,3 +1,4 @@
+//go:build circuitplay_express
 // +build circuitplay_express
 
 package main

src/examples/dac/pyportal.go

@@ -1,3 +1,4 @@
+//go:build pyportal
 // +build pyportal
 
 package main

src/examples/pininterrupt/circuitplay-express.go

@@ -1,3 +1,4 @@
+//go:build circuitplay_express
 // +build circuitplay_express
 
 package main

src/examples/pininterrupt/pca10040.go

@@ -1,3 +1,4 @@
+//go:build pca10040
 // +build pca10040
 
 package main

src/examples/pininterrupt/stm32.go

@@ -1,3 +1,4 @@
+//go:build stm32
 // +build stm32
 
 package main