This is the newest sdk. I have skipped 9.3 for now but someone can
certainly add it if they need it for some reason.
Also I added a generic "xcode" that will always point to the newest
xcode that is available in Nixpkgs.
This reverts commit 0a944b345e89ca0096974d168f49e1c6830c3fc2, reversing
changes made to 61733ed6ccde3427016720f2e0cd191d3d95152c.
I dislike these massive stdenv changes with unclear motivation,
especially when they involve gratuitous mass renames like NIX_CC ->
NIX_BINUTILS. The previous such rename (NIX_GCC -> NIX_CC) caused
months of pain, so let's not do that again.