compiler: add "target-cpu" and "target-features" attributes

This matches Clang, and with that, it adds support for inlining between
Go and C because LLVM only allows inlining if the "target-cpu" and
"target-features" string attributes match.

For example, take a look at the following code:

    // int add(int a, int b) {
    //   return a + b;
    // }
    import "C"

    func main() {
        println(C.add(3, 5))
    }

The 'add' function is not inlined into the main function before this
commit, but after it, it can be inlined and trivially be optimized to
`println(8)`.

transform/transform_test.go

@@ -21,6 +21,11 @@ import (
 
 var update = flag.Bool("update", false, "update transform package tests")
 
+var defaultTestConfig = &compileopts.Config{
+	Target:  &compileopts.TargetSpec{},
+	Options: &compileopts.Options{Opt: "2"},
+}
+
 // testTransform runs a transformation pass on an input file (pathPrefix+".ll")
 // and checks whether it matches the expected output (pathPrefix+".out.ll"). The
 // output is compared with a fuzzy match that ignores some irrelevant lines such