depends: major upgrade to darwin toolchain

tl;dr: Update to the latest stable toolchain and SDK for OSX without giving up
any backwards compatibility.

Switch to a new, better maintained fork of cctools:
https://github.com/tpoechtrager/cctools-port

I've forked this and will be working on it some as well:
https://github.com/theuni/cctools-port

This brings in:
cctools v862
ld64: v241.9

It also fixes 64bit builds, so there's no longer any need to use a 32bit clang.
Since clang is no longer tied to an old/crusty 32bit build, it's been upgraded
to the latest stable 3.5.

After updating to stable clang/linker/cctools, it's possible to use a more
recent SDK. The current SDK (10.7) through the most recent 10.10 have all been
built/tested successfully, both with and without 10.6 compatibility.

SDKs >= 10.9 use libc++ rather than libstdc++. This is verified working as well.

We should consider building against 10.10 while keeping 10.6 as the minimum
supported version as a next step. In theory, that allows users running newer
OSX versions to take advantage of new features without affecting those running
older versions.