Adapt gdc PKGBUILD for ALARM

Hi,
we ported gdc (the GNU D compiler) to ARM now and I think it'd be great to have gdc available in
Archlinux ARM.  GDC is a component of GCC (although not part of the official GCC sources yet) and the flags used to configure it must therefore be the same as the flags used to configure the system GCC.

The only changes I did compared to the upstream PKGBUILD are:
* Replaced the bugurl
* Added ALARM specific configure flags to match those used for GCC

Please note that building GDC always causes a complete GCC build with the "c, c++, d" languages enabled so it might take some time / require quite some resources. I'm not sure if the ```noautobuild=1``` flag should be set as it's done in the GCC PKGBUILD.