libafs: DARWIN: update for Xcode 5.1

(1) remove -mlong-branch from amd64 build

Random internet postings suggest that it has triggered a warning
since at least Xcode 3.2, and the gcc manual page suggests that
it is only applicable on ppc, anyway.

(2) remove -mpreferred-stack-boundary=4 from the amd64 build

The evidence here shows up less readily in an internet search,
but it seems that Apple's compilers will force the stack alignment
to 16 bytes regardless of what is passed here.  One poster had
trouble with -mpreferred-stack-boundary being unused in Xcode 4.4.1

This change only fixes warnings reported as errors by buildbot; it
does not attempt to fully synchronize with the flags that Xcode 5.1
uses for kernel module builds.

Change-Id: Iac1fcf3cdb5ab847a04278d1c05761bd371828e2
Reviewed-on: http://gerrit.openafs.org/10896
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>