targets: let specific targets inherit more general targets

This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.

Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).

targets/cortex-m.json

@@ -0,0 +1,7 @@
+{
+	"build-tags": ["tinygo.arm", "js", "wasm"],
+	"linker": "arm-none-eabi-gcc",
+	"rtlib": "compiler-rt",
+	"objcopy": "arm-none-eabi-objcopy",
+	"gdb": "arm-none-eabi-gdb"
+}