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

examples/fnv1a/asm.go

@@ -14,7 +14,7 @@ const (
 )
 
 func main() {
-	TEXT("Hash64", "func(data []byte) uint64")
+	TEXT("Hash64", NOSPLIT, "func(data []byte) uint64")
 	Doc("Hash64 computes the FNV-1a hash of data.")
 	ptr := Load(Param("data").Base(), GP64())
 	n := Load(Param("data").Len(), GP64())