perf tools: Add DWARF register lookup for SH

Implement get_arch_regstr() for SH so that, given a DWARF register number, the
corresponding symbolic name of that register can be looked up.

Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Ian Munsie <imunsie@au.ibm.com>
LKML-Reference: <e55812819ad18c2ceca5651ac7698a2af46180d7.1278774279.git.matt@console-pimps.org>
Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>