Add AVR support

This requires support in LLVM, as AVR support is still experimental. For
example, in bindings/go/build.sh, add
-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=AVR to cmake_flags.

src/device/avr/avr.go

@@ -0,0 +1,9 @@
+package avr
+
+// Magic type recognozed by the compiler to mark this string as inline assembly.
+type __asm string
+
+// Run the given assembly code. The code will be marked as having side effects,
+// as it doesn't produce output and thus would normally be eliminated by the
+// optimizer.
+func Asm(asm __asm)