Excludes ppc64 from kext support tools build.

Due to some as yet not understood bug, recent builds (>=V2.6.2)
of these tools have contained broken ppc64 content, resulting
in unusability on G5s.  This change excludes the broken content,
and has no effect on builds for >=10.6, where PPC is unsupported.

Although the bad builds (presumably under Xcode 3.2) are not
reproducible here, the effect of omitting the ppc64 "forks"
has been tested with Xcode 3.1.
Remove ppc7400 from support tools' VALID_ARCHS