kbuild: Cache a few more calls to the compiler

kbuild: Cache a few more calls to the compiler

These are a few stragglers that I left out of the original patch to
cache calls to the C compiler ("kbuild: Add a cache for generated
variables") because they bleed out into the main Makefile and thus
uglify things a little bit.  The idea is the same here, though.

Signed-off-by: Douglas Anderson <[email protected]>
Tested-by: Ingo Molnar <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>