all: make targets configurable with a JSON file

This is intentionally similar to the target specifications in Rust:
https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html

targets/arduino.json

@@ -0,0 +1,4 @@
+{
+	"llvm-target": "avr-atmel-none",
+	"build-tags": ["avr", "avr8", "atmega", "atmega328p", "js", "wasm"]
+}

targets/pca10040.json

@@ -0,0 +1,4 @@
+{
+	"llvm-target": "armv7m-none-eabi",
+	"build-tags": ["nrf", "nrf52", "nrf52832", "js", "wasm"]
+}