arm: use the lld linker

LLD version 8 has added support for armv6m:
    https://reviews.llvm.org/D55555
This means we can use LLD instead of arm-none-eabi-ld, eliminating our
dependency on GNU binutils.

There are small differences in code size, but never more than a few
bytes.

commands_macos.go

@@ -6,6 +6,6 @@ package main
 var commands = map[string]string{
 	"ar":      "llvm-ar",
 	"clang":   "clang-8",
-	"ld.lld":  "ld.lld-8",
+	"ld.lld":  "ld.lld",
 	"wasm-ld": "wasm-ld",
 }