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.go

@@ -21,11 +21,18 @@ import (
 // attributes to a function. For example, it adds optsize when requested from
 // the -opt= compiler flag.
 func AddStandardAttributes(fn llvm.Value, config *compileopts.Config) {
+	ctx := fn.Type().Context()
 	_, sizeLevel, _ := config.OptLevels()
 	if sizeLevel >= 1 {
-		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
+		fn.AddFunctionAttr(ctx.CreateEnumAttribute(llvm.AttributeKindID("optsize"), 0))
 	}
 	if sizeLevel >= 2 {
-		fn.AddFunctionAttr(fn.Type().Context().CreateEnumAttribute(llvm.AttributeKindID("minsize"), 0))
+		fn.AddFunctionAttr(ctx.CreateEnumAttribute(llvm.AttributeKindID("minsize"), 0))
+	}
+	if config.CPU() != "" {
+		fn.AddFunctionAttr(ctx.CreateStringAttribute("target-cpu", config.CPU()))
+	}
+	if config.Features() != "" {
+		fn.AddFunctionAttr(ctx.CreateStringAttribute("target-features", config.Features()))
 	}
 }