Align unused real alignment code to upstream source.

We set the target-specific values at runtime in Type::init().
Remove piece of dead code gone from upstream as well.

We do not support building with DMC anyway, so no
functionality change implied.
More IN_LLVM.
Default to PIC on OS X.

driver/target.h is due for a refactoring after the next
release is branched off.
Always add 'uwtable' attribute on x86_64.

This is what Clang does, and omitting it could lead to problems
w.r.t. the System V ABI. Doesn't seem to have caused any issues
so far, though.
Merge pull request #361 from klickverbot/osx

PIC by default on OS X; cleanups.