extern(D) should follow the extern(C) ABI on x86_64

[dnadlinger]

From http://dlang.org/abi.html: »The extern (C) and extern (D) calling convention matches the C calling convention used by the supported C compiler on the host system.« The argument order DMD uses seems to be still reversed compared to C, though.

Currently, a custom ABI modelled after the x86 D one is used on x86_64. This dates back to the days where no other D compiler really had x86_64 support.

Fixing this should mostly involve deleting code, but some issues might pop up here and there.

Oh, and keepUnchanged in gen/abi-x86-64.cpp should go, arrays and delegates are to be treated exactly like two element structs.