kbuild: builddeb - avoid recompiles for non-cross-compiles

Commit e2c318225ac1 ("kbuild: deb-pkg: add
pkg.linux-upstream.nokernelheaders build profile") changed how
install-extmod-build gets called, making it always rebuild the host
programs below scripts/ if HOSTCC wasn't specified with its full triplet
on the make command line. That is, apparently, needed to fix up commit
f1d87664b82a ("kbuild: cross-compile linux-headers package when
possible") for cross-compiles. However, in the much more common case of
non-cross-compile builds this will lead to unnecessary rebuilding of
host tools including gcc plugins. This, in turn, will lead to a full
kernel rebuild on the next 'make bindeb-pkg' which is unfortunate.

Avoid that by only triggering the rebuild of host tools for actual
cross-compile builds.

Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Fixes: e2c318225ac1 ("kbuild: deb-pkg: add pkg.linux-upstream.nokernelheaders build profile")
Cc: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Link: https://patch.msgid.link/20260402145116.1010901-1-minipli@grsecurity.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>