avr: use Clang for compiling C and assembly files

This is one step towards an avr-gcc free TinyGo toolchain, something
that will make it easier to use AVR targets in TinyGo.

targets/attiny85.json

@@ -3,7 +3,9 @@
 	"cpu": "attiny85",
 	"build-tags": ["attiny85", "attiny", "avr2", "avr25"],
 	"cflags": [
-		"-mmcu=attiny85"
+		"-mmcu=attiny85",
+		"-D__AVR_ARCH__=25",
+		"-Qunused-arguments"
 	],
 	"ldflags": [
 		"-mmcu=avr25"

targets/avr.S

@@ -2,6 +2,15 @@
 ; in Go.
 ; The reset vector is device-specific and is generated by tools/gen-device-avr.py.
 
+; These definitions are necessary because LLVM does not yet know these register
+; aliases. See: https://reviews.llvm.org/D96492
+#define xl r26
+#define xh r27
+#define yl r28
+#define yh r29
+#define zl r30
+#define zh r31
+
 ; Startup code
 .section .text.__vector_RESET
 .global  __vector_RESET

targets/avr.json

@@ -3,11 +3,14 @@
 	"build-tags": ["avr", "baremetal", "linux", "arm"],
 	"goos": "linux",
 	"goarch": "arm",
-	"compiler": "avr-gcc",
+	"compiler": "clang",
 	"gc": "conservative",
 	"linker": "avr-gcc",
 	"scheduler": "none",
 	"default-stack-size": 256,
+	"cflags": [
+		"--target=avr-unknown-unknown"
+	],
 	"ldflags": [
 		"-T", "targets/avr.ld",
 		"-Wl,--gc-sections"