ir,build: pragma support (#97)

Adds support for arbitrary compiler directives.

Fixes #15

examples/pragma/asm.go

@@ -0,0 +1,24 @@
+// +build ignore
+
+package main
+
+import (
+	. "github.com/mmcloughlin/avo/build"
+	. "github.com/mmcloughlin/avo/operand"
+)
+
+func main() {
+	TEXT("Add", NOSPLIT, "func(z, x, y *uint64)")
+	Pragma("noescape")
+	Doc("Add adds the values at x and y and writes the result to z.")
+	zptr := Mem{Base: Load(Param("z"), GP64())}
+	xptr := Mem{Base: Load(Param("x"), GP64())}
+	yptr := Mem{Base: Load(Param("y"), GP64())}
+	x, y := GP64(), GP64()
+	MOVQ(xptr, x)
+	MOVQ(yptr, y)
+	ADDQ(x, y)
+	MOVQ(y, zptr)
+	RET()
+	Generate()
+}