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/dot/asm.go

@@ -11,7 +11,7 @@ import (
 var unroll = 6
 
 func main() {
-	TEXT("Dot", "func(x, y []float32) float32")
+	TEXT("Dot", NOSPLIT, "func(x, y []float32) float32")
 	x := Mem{Base: Load(Param("x").Base(), GP64())}
 	y := Mem{Base: Load(Param("y").Base(), GP64())}
 	n := Load(Param("x").Len(), GP64())

examples/dot/dot.s

@@ -1,7 +1,9 @@
 // Code generated by command: go run asm.go -out dot.s -stubs stub.go. DO NOT EDIT.
 
+#include "textflag.h"
+
 // func Dot(x []float32, y []float32) float32
-TEXT ·Dot(SB), $0-52
+TEXT ·Dot(SB), NOSPLIT, $0-52
 	MOVQ	x_base(FP), AX
 	MOVQ	y_base+24(FP), CX
 	MOVQ	x_len+8(FP), DX