Bug 116131: tools/perf and O= fails to build

[TJ]

Trying to do an out-of-tree build using either absolute or relative 
paths for O= fails when trying to build tools/perf.

More details in ""Directory does not exist" when doing tools 
perf_install out-of-tree build"

https://bugzilla.kernel.org/show_bug.cgi?id=116131

git blame shows commits c883122a and bf35182f are responsible for the 
path calculation code.