gcc: Add support for label attributes and "unavailable" attribute.
Apple GCC has extensions to support for both label attributes and
an "unavailable" attribute. These are critical for objc but are
also useful in regular C/C++.
Obtained from: Apple GCC 4.2 - 5531
non-i386, non-unix, and generatable files have been trimmed, but can easily
be added in later if needed.
gcc-2.7.2.1 will follow shortly, it's a very small delta to this and it's
handy to have both available for reference for such little cost.
The freebsd-specific changes will then be committed, and once the dust has
settled, the bmakefiles will be committed to use this code.
