kbuild: cross-compile linux-headers package when possible

A long standing issue in the upstream kernel packaging is that the
linux-headers package is not cross-compiled.

For example, you can cross-build Debian packages for arm64 by running
the following command:

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg

However, the generated linux-headers-*_arm64.deb is useless because the
host programs in it were built for your build machine architecture
(likely x86), not arm64.

The Debian kernel maintains its own Makefiles to cross-compile host
tools without relying on Kbuild. [1]

Instead of adding such full custom Makefiles, this commit adds a small
piece of code to cross-compile host programs located under the scripts/
directory.

A straightforward solution is to pass HOSTCC=${CROSS_COMPILE}gcc, but it
would also cross-compile scripts/basic/fixdep, which needs to be native
to process the if_changed_dep macro. (This approach may work under some
circumstances; you can execute foreign architecture programs with the
help of binfmt_misc because Debian systems enable CONFIG_BINFMT_MISC,
but it would require installing QEMU and libc for that architecture.)

A trick is to use the external module build (KBUILD_EXTMOD=), which
does not rebuild scripts/basic/fixdep. ${CC} needs to be able to link
userspace programs (CONFIG_CC_CAN_LINK=y).

There are known limitations:

 - GCC plugins

   It would possible to rebuild GCC plugins for the target architecture
   by passing HOSTCXX=${CROSS_COMPILE}g++ with necessary packages
   installed, but gcc on the installed system emits
   "cc1: error: incompatible gcc/plugin versions".

 - objtool and resolve_btfids

   These are built by the tools build system. They are not covered by
   the current solution. The resulting linux-headers package is broken
   if CONFIG_OBJTOOL or CONFIG_DEBUG_INFO_BTF is enabled.

I only tested this with Debian, but it should work for other package
systems as well.

[1]: https://salsa.debian.org/kernel-team/linux/-/blob/debian/6.9.9-1/debian/rules.real#L586

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>

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