Change the "make clean" rule to remove all the .tmp_* files.

.tmp_objdiff is the only exception, which should be removed by
"make mrproper".

Rename the record directory of objdiff, .tmp_objdiff to .objdiff to
avoid the removal by "make clean".
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Sedat Dilek <sedat.dilek@gmail.com> # LLVM-14 (x86-64)