Remove -fno-guess-branch-probability and -fno-unit-at-a-time.

bde enabled -fno-guess-branch-probability in 2003, well before our
current compiler was imported. At the time it produced weirdly orded
code. It no longer does that. It also saves 0-4 bytes depending on
other options.

kan disabled unit-at-a-time in 2004 because it badly mangled boot2 so
it wouldn't work. That too was before the 4.2.1 compiler, where it no
longer does that. This saves 44 bytes.

I had planned to document why they were needed, but when I discovered
their antiquity, I removed them and boot2 still works and is
smaller. In qemu, the old and new boot2's behaved identically.

These are gcc specific hacks, and won't affect clang-built boot2
at all.