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())

examples/fnv1a/fnv1a.s

@@ -1,7 +1,9 @@
 // Code generated by command: go run asm.go -out fnv1a.s -stubs stub.go. DO NOT EDIT.
 
+#include "textflag.h"
+
 // func Hash64(data []byte) uint64
-TEXT ·Hash64(SB), $0-32
+TEXT ·Hash64(SB), NOSPLIT, $0-32
 	MOVQ	data_base(FP), CX
 	MOVQ	data_len+8(FP), BX
 	MOVQ	$0xcbf29ce484222325, AX