targets: add support for GameBoy Advance

Only tested in an emulator (mGBA). Almost nothing is supported, but
drawing to the screen works.

targets/gameboy-advance.json

@@ -0,0 +1,29 @@
+{
+	"llvm-target": "thumb4-none-eabi",
+	"cpu": "arm7tdmi",
+	"build-tags": ["gameboyadvance", "arm7tdmi", "baremetal", "linux", "arm"],
+	"goos": "linux",
+	"goarch": "arm",
+	"compiler": "clang",
+	"linker": "ld.lld",
+	"cflags": [
+		"-g",
+		"--target=thumb4-none-eabi",
+		"-mcpu=arm7tdmi",
+		"-Oz",
+		"-Werror",
+		"-fshort-enums",
+		"-Wno-macro-redefined",
+		"-Qunused-arguments",
+		"-fno-exceptions", "-fno-unwind-tables",
+		"-ffunction-sections", "-fdata-sections"
+	],
+	"ldflags": [
+		"--gc-sections",
+		"-Ttargets/gameboy-advance.ld"
+	],
+	"extra-files": [
+		"targets/gameboy-advance.s"
+	],
+	"emulator": ["mgba-qt"]
+}