Kbuild, lto: Set TMPDIR for LTO v4

LTO gcc puts a lot of data into $TMPDIR, essentially another copy
of the object directory to pass the repartitioned object files
to the code generation processes.

TMPDIR defaults to /tmp With /tmp as tmpfs it's easy to drive systems to
out of memory, because they will compete with the already high anonymous
memory consumption of the wpa LTO pass.

When LTO is set always set TMPDIR to the object directory. This could
be slightly slower, but is far safer and eliminates another parameter
the LTO user would need to set manually.

I made it conditional on LTO for now.

v2: Allow user to override (H. Peter Anvin)
v3: Use standard kernel variable style
v4: Print message for redirection (M.Marek)
Signed-off-by: Andi Kleen <ak@linux.intel.com>