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.

avr.S

@@ -0,0 +1,62 @@
+.section .isr
+isr:
+    rjmp reset
+
+.org 0x18 ; WDT
+    rjmp wdt
+
+; Startup code
+.section .reset
+.org 26
+reset:
+    clr  r1          ; r1 is expected to be 0 by the C calling convention
+
+    ; Zero .bss
+clear_bss:
+    ldi  xl, lo8(_sbss)
+    ldi  xh, hi8(_sbss)
+clear_bss_loop:
+    ldi  yl, lo8(_ebss)
+    ldi  yh, hi8(_ebss)
+    cp   xl, yl         ; if x == y
+    cpc  xh, yh
+    breq clear_bss_end
+    st   x+, r1         ; zero byte in *x
+    rjmp clear_bss_loop
+clear_bss_end:
+
+    ; Set up the stack pointer.
+    ldi  xl, lo8(_stack_top)
+    ldi  xh, hi8(_stack_top)
+    out  0x3d, xl; SPL
+    out  0x3e, xh; SPH
+
+    ; Enable interrupts.
+    ; TODO: make sure interrupts are started after all initializers have run.
+    sei
+
+    ; main will be placed right after here by the linker script so there's no
+    ; need to jump.
+
+
+; The only thing this WDT handler really does is disable itself, to get out of
+; sleep mode.
+.section .text.wdt
+wdt:
+    push r16
+
+    clr  r16
+    wdr            ; Reset watchdog
+    out  0x34, r16 ; Clear reset reason (MCUSR)
+
+    ; part 1: set WDCE and WDE to enable editing WDTCSR
+    lds  r16, 0x60 ; r16 = WDTCSR
+    ori  r16, 0x18 ; r16 |= WDCE | WDE
+    sts  0x60, r16 ; WDTCSR = r16
+
+    ; part 2: within 4 clock cycles, set the new value for WDTCSR
+    clr  r16
+    sts  0x60, r16 ; WDTCSR = 0
+
+    pop  r16
+    reti