nintendoswitch: support outputting .nro files directly

By modifying the linker script a bit and adding the NRO0 header directly
in the assembly, it's possible to craft an ELF file that can be
converted straight to a binary (using objcopy or similar) that is a NRO
file. This avoids custom code for NRO files or an extra build step.

With another change, .nro files are recognized by TinyGo so that this
will create a ready-to-run NRO file:

    tinygo build -o test.nro -target=nintendoswitch examples/serial

compileopts/config.go

@@ -243,7 +243,7 @@ func (c *Config) Debug() bool {
 // extension and the configured binary format in the target JSON file.
 func (c *Config) BinaryFormat(ext string) string {
 	switch ext {
-	case ".bin", ".gba":
+	case ".bin", ".gba", ".nro":
 		// The simplest format possible: dump everything in a raw binary file.
 		if c.Target.BinaryFormat != "" {
 			return c.Target.BinaryFormat