perf currently fails to build on MIPS as there is no
tools/perf/arch/mips/Build file.  Adding an empty file fixes this as
there are no MIPS-specific sources to build.

It looks like the same is needed for Alpha and PA-RISC, though I
haven't been able to test those.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Fixes: 5e8c0fb6 ("perf build: Add arch x86 objects building")
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1438704627.7315.2.camel@decadent.org.ukSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>