perf tools: Allow to specify custom linker command

Allow to specify custom linker command. This fixes MIPS64 builds for
64-bit userspace as it will allow to pass a linker using the correct
linker flags for 64-bit ABI (by default GNU binutils ld will assume
N32).

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1435751683-18500-2-git-send-email-aaro.koskinen@nokia.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>