cgo: support builtin #include headers

Add support for header files bundled with the compiler by copying them
into the release tarball.

targets/cortex-m.json

@@ -9,7 +9,10 @@
 	"cflags": [
 		"-Oz",
 		"-mthumb",
+		"-Werror",
 		"-fshort-enums",
+		"-nostdlibinc",
+		"-Wno-macro-redefined",
 		"-fno-exceptions", "-fno-unwind-tables",
 		"-ffunction-sections", "-fdata-sections"
 	],

targets/wasm.json

@@ -7,6 +7,8 @@
 	"linker":        "wasm-ld",
 	"cflags": [
 		"--target=wasm32",
+		"-nostdlibinc",
+		"-Wno-macro-redefined",
 		"-Oz"
 	],
 	"ldflags": [