rfmon.o CONFIG_E500 specific

Subject says it all, there is no need to link perfmon.o on
sub-architectures other than CONFIG_E500.

Signed-off-by: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
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