all: support Arduino in the compiler driver

This makes sure the compiler itself can build/link an ELF file that
works on an Arduino.

main.go

@@ -89,7 +89,8 @@ func Compile(pkgName, runtimePath, outpath, target string, printIR, dumpSSA bool
 
 		// Link the object file with the system compiler.
 		executable := filepath.Join(dir, "main")
-		cmd := exec.Command("cc", "-o", executable, objfile)
+		args := append(spec.PreLinkArgs, "-o", executable, objfile)
+		cmd := exec.Command(spec.Linker, args...)
 		cmd.Stdout = os.Stdout
 		cmd.Stderr = os.Stderr
 		err = cmd.Run()

target.go

@@ -16,6 +16,8 @@ import (
 type TargetSpec struct {
 	Triple      string   `json:"llvm-target"`
 	BuildTags   []string `json:"build-tags"`
+	Linker      string   `json:"linker"`
+	PreLinkArgs []string `json:"pre-link-args"`
 }
 
 // Load a target specification
@@ -23,6 +25,7 @@ func LoadTarget(target string) (*TargetSpec, error) {
 	spec := &TargetSpec{
 		Triple:    target,
 		BuildTags: []string{runtime.GOOS, runtime.GOARCH},
+		Linker:    "cc",
 	}
 
 	// See whether there is a target specification for this target (e.g.

targets/arduino.json

@@ -1,4 +1,6 @@
 {
 	"llvm-target": "avr-atmel-none",
-	"build-tags": ["avr", "avr8", "atmega", "atmega328p", "js", "wasm"]
+	"build-tags": ["avr", "avr8", "atmega", "atmega328p", "js", "wasm"],
+	"linker": "avr-gcc",
+	"pre-link-args": ["-nostdlib", "-T", "avr.ld", "-Wl,--gc-sections", "avr.S"]
 }