strace: unavailable for AArch64 and ARC

strace isn't available for the AArch64 and ARC architectures.
The patchset for AArch64 is somewhat big and complicated (needs updates
to other bits) so it'd probably be better to use a git version of strace
or wait for a new release.
http://autobuild.buildroot.net/results/506f4adec348f0b616ad09bddbcbc242e38253b8/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>