kbuild: make $(checker-shell ) strip spaces around the result

It looks like GNU make version 3.80 (but apparently not 3.81) adds
leading whitespace to the result of the checker-shell execution.  This
strips them off explicitly.

Also, don't bother symlinking the output file to /dev/null.  It's likely
as expensive as just writing the temp-file, and we need to remove it
anyway afterwards.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>