build: add attributes argument to TEXT

Brings the TEXT() function signature closer to how the directive works
in assembly.

Also update almost all examples to use NOSPLIT.

Closes #11

tests/cast/asm.go

@@ -7,7 +7,7 @@ import (
 )
 
 func main() {
-	TEXT("Split", "func(x uint64) (q uint64, l uint32, w uint16, b uint8)")
+	TEXT("Split", NOSPLIT, "func(x uint64) (q uint64, l uint32, w uint16, b uint8)")
 	Doc(
 		"Split returns the low 64, 32, 16 and 8 bits of x.",
 		"Tests the As() methods of virtual general-purpose registers.",