gcc: fix collect2 host contamination problem properly

gcc: fix collect2 host contamination problem properly

We added the autoconf cache line a while back to ensure that configure doesn't
poke into some hardcoded host paths looking for things it shouldn't. Applying
it as part of do_configure wasn't getting it to the do_compile tasks
where much of the configure scripts are run by gcc. This changes it to a simple
export to ensure it reaches the places it needs to and truly gets rid of
the cross compile badness messages from the logs.

(From OE-Core rev: af58d5c3a5c5e632aa20b1d007d47dbbfb4ed6e6)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>