perf tool: Fix gcc 4.6.0 issues

GCC 4.6.0 in Fedora rawhide turned up some compile errors in tools/perf
due to the -Werror=unused-but-set-variable flag.

I've gone through and annotated some of the assignments that had side
effects (ie: return value from a function) with the __used annotation,
and in some cases, just removed unused code.

In a few cases, we were assigning something useful, but not using it in
later parts of the function.

kyle@dreadnought:~/src% gcc --version
gcc (GCC) 4.6.0 20110122 (Red Hat 4.6.0-0.3)

Cc: Ingo Molnar <mingo@redhat.com>
LKML-Reference: <20110124161304.GK27353@bombadil.infradead.org>
Signed-off-by: Kyle McMartin <kyle@redhat.com>
[ committer note: Fixed up the annotation fixes, as that code moved recently ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>