targets: simulate Circuit Playground Express in play.tinygo.org

This requires a number of changes to atsamd21 code. It's not a very
clean separation, but I couldn't come up with a better one.

src/machine/machine_atsamd21.go

@@ -14,8 +14,6 @@ import (
 	"unsafe"
 )
 
-const CPU_FREQUENCY = 48000000
-
 type PinMode uint8
 
 const (
@@ -35,74 +33,6 @@ const (
 	PinInputPulldown PinMode = 12
 )
 
-// Hardware pins
-const (
-	PA00 Pin = 0
-	PA01 Pin = 1
-	PA02 Pin = 2
-	PA03 Pin = 3
-	PA04 Pin = 4
-	PA05 Pin = 5
-	PA06 Pin = 6
-	PA07 Pin = 7
-	PA08 Pin = 8
-	PA09 Pin = 9
-	PA10 Pin = 10
-	PA11 Pin = 11
-	PA12 Pin = 12
-	PA13 Pin = 13
-	PA14 Pin = 14
-	PA15 Pin = 15
-	PA16 Pin = 16
-	PA17 Pin = 17
-	PA18 Pin = 18
-	PA19 Pin = 19
-	PA20 Pin = 20
-	PA21 Pin = 21
-	PA22 Pin = 22
-	PA23 Pin = 23
-	PA24 Pin = 24
-	PA25 Pin = 25
-	PA26 Pin = 26
-	PA27 Pin = 27
-	PA28 Pin = 28
-	PA29 Pin = 29
-	PA30 Pin = 30
-	PA31 Pin = 31
-	PB00 Pin = 32
-	PB01 Pin = 33
-	PB02 Pin = 34
-	PB03 Pin = 35
-	PB04 Pin = 36
-	PB05 Pin = 37
-	PB06 Pin = 38
-	PB07 Pin = 39
-	PB08 Pin = 40
-	PB09 Pin = 41
-	PB10 Pin = 42
-	PB11 Pin = 43
-	PB12 Pin = 44
-	PB13 Pin = 45
-	PB14 Pin = 46
-	PB15 Pin = 47
-	PB16 Pin = 48
-	PB17 Pin = 49
-	PB18 Pin = 50
-	PB19 Pin = 51
-	PB20 Pin = 52
-	PB21 Pin = 53
-	PB22 Pin = 54
-	PB23 Pin = 55
-	PB24 Pin = 56
-	PB25 Pin = 57
-	PB26 Pin = 58
-	PB27 Pin = 59
-	PB28 Pin = 60
-	PB29 Pin = 61
-	PB30 Pin = 62
-	PB31 Pin = 63
-)
-
 const (
 	pinPadMapSERCOM0Pad0 byte = (0x10 << 1) | 0x00
 	pinPadMapSERCOM1Pad0 byte = (0x20 << 1) | 0x00