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.

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

src/examples/pininterrupt/wioterminal.go

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

src/examples/pwm/arduino-mega1280.go

@@ -1,3 +1,4 @@
+//go:build arduino_mega1280
 // +build arduino_mega1280
 
 package main

src/examples/pwm/arduino.go

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

src/examples/pwm/bluepill.go

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

src/examples/pwm/feather-m4.go

@@ -1,3 +1,4 @@
+//go:build feather_m4
 // +build feather_m4
 
 package main

src/examples/pwm/itsybitsy-m0.go

@@ -1,3 +1,4 @@
+//go:build itsybitsy_m0
 // +build itsybitsy_m0
 
 package main

src/examples/pwm/itsybitsy-m4.go

@@ -1,3 +1,4 @@
+//go:build itsybitsy_m4
 // +build itsybitsy_m4
 
 package main

src/examples/pwm/nucleo-f722ze.go

@@ -1,3 +1,4 @@
+//go:build stm32f7
 // +build stm32f7
 
 package main

src/examples/pwm/nucleo-l031k6.go

@@ -1,3 +1,4 @@
+//go:build stm32l0
 // +build stm32l0
 
 package main

src/examples/pwm/nucleo-l432kc.go

@@ -1,3 +1,4 @@
+//go:build stm32l4
 // +build stm32l4
 
 package main

src/examples/pwm/nucleo-l552ze.go

@@ -1,3 +1,4 @@
+//go:build stm32l5
 // +build stm32l5
 
 package main

src/examples/pwm/pico.go

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

src/examples/pwm/stm32f4disco.go

@@ -1,3 +1,4 @@
+//go:build stm32f4disco
 // +build stm32f4disco
 
 package main