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

build/attr.go

@@ -0,0 +1,17 @@
+package build
+
+import "github.com/mmcloughlin/avo/attr"
+
+// TEXT and DATA attribute values included for convenience.
+var (
+	NOPROF        = attr.NOPROF
+	DUPOK         = attr.DUPOK
+	NOSPLIT       = attr.NOSPLIT
+	RODATA        = attr.RODATA
+	NOPTR         = attr.NOPTR
+	WRAPPER       = attr.WRAPPER
+	NEEDCTXT      = attr.NEEDCTXT
+	TLSBSS        = attr.TLSBSS
+	NOFRAME       = attr.NOFRAME
+	REFLECTMETHOD = attr.REFLECTMETHOD
+)

build/global.go

@@ -15,9 +15,10 @@ import (
 // ctx provides a global build context.
 var ctx = NewContext()
 
-// TEXT starts building a new function called name and sets its signature (see SignatureExpr).
-func TEXT(name, signature string) {
+// TEXT starts building a new function called name, with attributes a, and sets its signature (see SignatureExpr).
+func TEXT(name string, a attr.Attribute, signature string) {
 	ctx.Function(name)
+	ctx.Attributes(a)
 	ctx.SignatureExpr(signature)
 }