build: unbreak ustack on smartos

I disabled the -ffunction-sections and -fdata-sections switches in 202df30a
because they're horribly buggy with some gcc/binutils combos.

However, it turns out that the dtrace/ustack post-processing tool requires
that V8 is compiled with said switches and was broken because of it.

This commit turns them on again on SunOS systems. Let's hope for the best.