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/sum/asm.go

@@ -8,7 +8,7 @@ import (
 )
 
 func main() {
-	TEXT("Sum", "func(xs []uint64) uint64")
+	TEXT("Sum", NOSPLIT, "func(xs []uint64) uint64")
 	Doc("Sum returns the sum of the elements in xs.")
 	ptr := Load(Param("xs").Base(), GP64())
 	n := Load(Param("xs").Len(), GP64())

examples/sum/sum.s

@@ -1,7 +1,9 @@
 // Code generated by command: go run asm.go -out sum.s -stubs stub.go. DO NOT EDIT.
 
+#include "textflag.h"
+
 // func Sum(xs []uint64) uint64
-TEXT ·Sum(SB), $0-32
+TEXT ·Sum(SB), NOSPLIT, $0-32
 	MOVQ	xs_base(FP), AX
 	MOVQ	xs_len+8(FP), CX
 	XORQ	DX, DX