main: add support for debugging qemu-user targets

This commit allows debugging like the following:

    GOARCH=arm tinygo gdb ./testdata/alias.go

This can be very useful to debug issues on a different instruction set
architecture but still on a host system.

I tested the following 7 configurations to make sure it works and I
didn't break anything:

    GOOS=amd64
    GOOS=386
    GOOS=arm
    GOOS=arm64
    tinygo gdb -target=hifive1-qemu
    tinygo gdb -target=cortex-m-qemu
    tinygo gdb -target=microbit

compileopts/target.go

@@ -240,7 +240,6 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 		CFlags:    []string{"--target=" + triple},
 		GDB:       "gdb",
 		PortReset: "false",
-		FlashMethod: "native",
 	}
 	if goos == "darwin" {
 		spec.LDFlags = append(spec.LDFlags, "-Wl,-dead_strip")
@@ -249,16 +248,15 @@ func defaultTarget(goos, goarch, triple string) (*TargetSpec, error) {
 	}
 	if goarch != runtime.GOARCH {
 		// Some educated guesses as to how to invoke helper programs.
+		spec.GDB = "gdb-multiarch"
 		if goarch == "arm" && goos == "linux" {
 			spec.CFlags = append(spec.CFlags, "--sysroot=/usr/arm-linux-gnueabihf")
 			spec.Linker = "arm-linux-gnueabihf-gcc"
-			spec.GDB = "arm-linux-gnueabihf-gdb"
 			spec.Emulator = []string{"qemu-arm", "-L", "/usr/arm-linux-gnueabihf"}
 		}
 		if goarch == "arm64" && goos == "linux" {
 			spec.CFlags = append(spec.CFlags, "--sysroot=/usr/aarch64-linux-gnu")
 			spec.Linker = "aarch64-linux-gnu-gcc"
-			spec.GDB = "aarch64-linux-gnu-gdb"
 			spec.Emulator = []string{"qemu-aarch64", "-L", "/usr/aarch64-linux-gnu"}
 		}
 		if goarch == "386" && runtime.GOARCH == "amd64" {

main.go

@@ -351,13 +351,17 @@ func FlashGDB(pkgName string, ocdOutput bool, options *compileopts.Options) erro
 				// Assume QEMU as an emulator.
 				if config.Target.Emulator[0] == "mgba" {
 					gdbInterface = "mgba"
-				} else {
+				} else if strings.HasPrefix(config.Target.Emulator[0], "qemu-system-") {
 					gdbInterface = "qemu"
+				} else {
+					gdbInterface = "qemu-user"
 				}
 			} else if openocdInterface != "" && config.Target.OpenOCDTarget != "" {
 				gdbInterface = "openocd"
 			} else if config.Target.JLinkDevice != "" {
 				gdbInterface = "jlink"
+			} else {
+				gdbInterface = "native"
 			}
 		}
 
@@ -409,6 +413,14 @@ func FlashGDB(pkgName string, ocdOutput bool, options *compileopts.Options) erro
 			daemon = exec.Command(config.Target.Emulator[0], args...)
 			daemon.Stdout = os.Stdout
 			daemon.Stderr = os.Stderr
+		case "qemu-user":
+			gdbCommands = append(gdbCommands, "target remote :1234")
+
+			// Run in an emulator.
+			args := append(config.Target.Emulator[1:], "-g", "1234", result.Binary)
+			daemon = exec.Command(config.Target.Emulator[0], args...)
+			daemon.Stdout = os.Stdout
+			daemon.Stderr = os.Stderr
 		case "mgba":
 			gdbCommands = append(gdbCommands, "target remote :2345")