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.ld

@@ -0,0 +1,43 @@
+
+MEMORY
+{
+    FLASH_TEXT (rw) : ORIGIN = 0, LENGTH = 32256
+    RAM (xrw)       : ORIGIN = 0, LENGTH = 2K
+}
+
+_stack_size = 512;
+
+SECTIONS
+{
+    .text :
+    {
+        KEEP(*(.isr))
+        KEEP(*(.reset))
+        KEEP(*(.text.main)) /* main must follow the reset handler */
+        *(.text.*)
+        *(.rodata)
+        *(.rodata.*)
+    }
+
+    .stack :
+    {
+        . += _stack_size;
+        _stack_top = .;
+    } >RAM
+
+    .data :
+    {
+        *(.data)
+        *(.data*)
+    } >RAM AT>FLASH_TEXT
+
+    .bss :
+    {
+        _sbss = .;
+        *(.bss)
+        *(.bss*)
+        *(COMMON)
+        _ebss = .;
+        _heap_start = .;
+    } >RAM
+}