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/geohash/asm.go

@@ -8,7 +8,7 @@ import (
 )
 
 func main() {
-	TEXT("EncodeInt", "func(lat, lng float64) uint64")
+	TEXT("EncodeInt", NOSPLIT, "func(lat, lng float64) uint64")
 	Doc("EncodeInt computes the 64-bit integer geohash of (lat, lng).")
 	lat := Load(Param("lat"), XMM())
 	lng := Load(Param("lng"), XMM())

examples/geohash/geohash.s

@@ -3,7 +3,7 @@
 #include "textflag.h"
 
 // func EncodeInt(lat float64, lng float64) uint64
-TEXT ·EncodeInt(SB), $0-24
+TEXT ·EncodeInt(SB), NOSPLIT, $0-24
 	MOVSD	lat(FP), X0
 	MOVSD	lng+8(FP), X1
 	MULSD	reciprocal180<>(SB), X0