ast,build: only include textflag.h if required

[mmcloughlin]

We currently always include textflag.h

avo/printer/goasm.go

Lines 39 to 43 in e364d63

	func (p *goasm) header() {
	p.Comment(p.cfg.GeneratedWarning())
	p.NL()
	p.include("textflag.h")
	}

We should be able to write a simple pass which uses ContainsTextFlags() to determine if a given file needs the include or not.

avo/attr.go

Lines 62 to 63 in e364d63

	// ContainsTextFlags() returns whether the Asm() representation requires macros in "textflags.h".
	func (a Attribute) ContainsTextFlags() bool {