perf tools: Fix build error on read only source.

perf tools: Fix build error on read only source.

Parts of the build process were generating files outside the specified
O= directory, causing the build to fail on systems where the sources are
in a read only file system.

Fix it by using $(OUTPUT) on these locations.

Also check that $(OUTPUT) actually exists, just like the top level
kernel Makefile does. Otherwise the failure message emitted is
completely misleading.

Cc: Ingo Molnar <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Paul Mackerras <[email protected]>
LKML-Reference: <20100817140841.0859362C03A@msa106.auone-net.jp>
Signed-off-by: Kusanagi Kouichi <[email protected]>
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>