main: use OS specific name for llvm-ar-7 tool to ensure that llvm7 toolchain works as expected

Signed-off-by: Ron Evans <ron@hybridgroup.com>

commands.go

@@ -0,0 +1,11 @@
+// +build !darwin
+
+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-7",
+	"clang":   "clang-7",
+	"ld.lld":  "ld.lld-7",
+	"wasm-ld": "wasm-ld-7",
+}