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

commands.go

@@ -4,7 +4,6 @@ package main
 
 // commands used by the compilation process might have different file names on Linux than those used on macOS.
 var commands = map[string]string{
-	"ar":      "llvm-ar-8",
 	"clang":   "clang-8",
 	"ld.lld":  "ld.lld-8",
 	"wasm-ld": "wasm-ld-8",