kbuild: refactor cross-compiling linux-headers package

kbuild: refactor cross-compiling linux-headers package

Since commit 13b25489b6f8 ("kbuild: change working directory to external
module directory with M="), when cross-building host programs for the
linux-headers package, the "Entering directory" and "Leaving directory"
messages appear multiple times, and each object path shown is relative
to the working directory. This makes it difficult to track which objects
are being rebuilt.

In hindsight, using the external module build (M=) was not a good idea.

This commit simplifies the script by leveraging the run-command target,
resulting in a cleaner build log again.

[Before]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
    [ snip ]
  Rebuilding host programs with aarch64-linux-gnu-gcc...
  make[5]: Entering directory '/home/masahiro/linux'
  make[6]: Entering directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
    HOSTCC  scripts/kallsyms
    HOSTCC  scripts/sorttable
    HOSTCC  scripts/asn1_compiler
  make[6]: Leaving directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
  make[5]: Leaving directory '/home/masahiro/linux'
  make[5]: Entering directory '/home/masahiro/linux'
  make[6]: Entering directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
    HOSTCC  scripts/basic/fixdep
    HOSTCC  scripts/mod/modpost.o
    HOSTCC  scripts/mod/file2alias.o
    HOSTCC  scripts/mod/sumversion.o
    HOSTCC  scripts/mod/symsearch.o
    HOSTLD  scripts/mod/modpost
  make[6]: Leaving directory '/home/masahiro/linux/debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+'
  make[5]: Leaving directory '/home/masahiro/linux'

[After]

  $ make ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- bindeb-pkg
    [ snip ]
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/basic/fixdep
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/kallsyms
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/sorttable
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/asn1_compiler
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/modpost.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/file2alias.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/sumversion.o
    HOSTCC  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/symsearch.o
    HOSTLD  debian/linux-headers-6.13.0-rc1+/usr/src/linux-headers-6.13.0-rc1+/scripts/mod/modpost

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>