ir,build: pragma support (#97)

Adds support for arbitrary compiler directives.

Fixes #15

examples/pragma/stub.go

@@ -0,0 +1,7 @@
+// Code generated by command: go run asm.go -out pragma.s -stubs stub.go. DO NOT EDIT.
+
+package pragma
+
+// Add adds the values at x and y and writes the result to z.
+//go:noescape
+func Add(z *uint64, x *uint64, y *uint64)