avr: use precise GC by default

AVR is especially sensitive to false positives in the GC for a few
reasons:

  * Pointer values are only two bytes, instead of four on most other
    microcontrollers.
  * Pointers are not aligned, so any two-byte value in an object coud be
    a pointer.
  * Memory typically starts at a low address, which makes it much more
    likely to clash with a regular integer value (which tend to be small
    values).

Therefore, use the precise GC (instead of the conservative GC) by
default. This increases binary size by around 350 bytes, but I think
it's a good tradeoff in most cases: it avoids some runtime errors and
generally RAM tends to be a lot more scarce than flash on AVR chips. If
this is too much, `-gc=conservative` or `-gc=leaking` can be used
instead.

main_test.go

@@ -210,10 +210,6 @@ func runPlatTests(options compileopts.Options, tests []string, t *testing.T) {
 				// limited amount of memory.
 				continue
 
-			case "gc.go":
-				// Does not pass due to high mark false positive rate.
-				continue
-
 			case "json.go", "stdlib.go", "testing.go":
 				// Too big for AVR. Doesn't fit in flash/RAM.
 				continue