main: optionally build with LLD

When building statically against LLVM, LLD is also included now. When
included, the built in wasm-ld will automatically be used instead of the
external command.

There is also support for linking ELF files but because lld does not
fully support armv6m this is not yet enabled (it produces a warning).

main.go

@@ -21,8 +21,10 @@ import (
 )
 
 var commands = map[string]string{
-	"ar":    "ar",
-	"clang": "clang-7",
+	"ar":      "ar",
+	"clang":   "clang-7",
+	"ld.lld":  "ld.lld-7",
+	"wasm-ld": "wasm-ld-7",
 }
 
 // commandError is an error type to wrap os/exec.Command errors. This provides
@@ -234,11 +236,7 @@ func Compile(pkgName, outpath string, spec *TargetSpec, config *BuildConfig, act
 		}
 
 		// Link the object files together.
-		cmd := exec.Command(spec.Linker, ldflags...)
-		cmd.Stdout = os.Stdout
-		cmd.Stderr = os.Stderr
-		cmd.Dir = sourceDir()
-		err = cmd.Run()
+		err = Link(sourceDir(), spec.Linker, ldflags...)
 		if err != nil {
 			return &commandError{"failed to link", executable, err}
 		}