Darwin: Don't use ARCHFLAGS to determine arch

Darwin: Don't use ARCHFLAGS to determine arch

The Darwin build uses ARCHFLAGS to control the architectures it
builds binaries for. However, this causes problems for standalone
builds which just do ./configure && make. So, if ARCHFLAGS is
undefined, ask gcc which architecture it thinks that it is
building for, and use that.

Change-Id: If9e7dc3c41ded5ac0593ae20ddd741690914a699
Reviewed-on: http://gerrit.openafs.org/1181
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>