main: drop the dependency on llvm-ar

The ar file format is pretty simple and can be implemented by using a Go
library. Use that instead of calling out to llvm-ar.

There are a few limitations to the used package, but that doesn't seem
to matter for our use case (linking compiler-rt for use with ld.lld):

  * no index is created
  * long filenames are truncated
  * no support for archives bigger than 4GB

Gopkg.lock

@@ -1,6 +1,14 @@
 # This file is autogenerated, do not edit; changes may be undone by the next 'dep ensure'.
 
 
+[[projects]]
+  branch = "master"
+  digest = "1:06519a2ec1d59040eaccec40206f9d0b59dc662db2a032f974d6d6b9a2bcb839"
+  name = "github.com/blakesmith/ar"
+  packages = ["."]
+  pruneopts = "UT"
+  revision = "8bd4349a67f2533b078dbc524689d15dba0f4659"
+
 [[projects]]
   branch = "master"
   digest = "1:00b45e06c7843541372fc17d982242bd6adfc2fc382b6f2e9ef9ce53d87a50b9"
@@ -33,6 +41,7 @@
   analyzer-name = "dep"
   analyzer-version = 1
   input-imports = [
+    "github.com/blakesmith/ar",
     "github.com/marcinbor85/gohex",
     "golang.org/x/tools/go/ast/astutil",
     "golang.org/x/tools/go/ssa",