kbuild: make all file references relative to source root

-fmacro-prefix-map only affects __FILE__ and __BASE_FILE__.
Other references, for example in debug information, are not affected.
This makes handling of file references in the compiler outputs harder to
use and creates problems for reproducible builds.

Switch to -ffile-prefix map which affects all references.

Also drop the documentation section advising manual specification of
-fdebug-prefix-map for reproducible builds, as it is not necessary
anymore.

Suggested-by: Ben Hutchings <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Thomas Weißschuh <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/reproducible-builds.rst

···
 `KBUILD_BUILD_USER and KBUILD_BUILD_HOST`_ variables.  If you are
 building from a git commit, you could use its committer address.
 
-Absolute filenames
-------------------
-
-When the kernel is built out-of-tree, debug information may include
-absolute filenames for the source files.  This must be overridden by
-including the ``-fdebug-prefix-map`` option in the `KCFLAGS`_ variable.
-
-Depending on the compiler used, the ``__FILE__`` macro may also expand
-to an absolute filename in an out-of-tree build.  Kbuild automatically
-uses the ``-fmacro-prefix-map`` option to prevent this, if it is
-supported.
-
-The Reproducible Builds web site has more information about these
-`prefix-map options`_.
-
 Generated files in source packages
 ----------------------------------
 
···
 
 .. _KBUILD_BUILD_TIMESTAMP: kbuild.html#kbuild-build-timestamp
 .. _KBUILD_BUILD_USER and KBUILD_BUILD_HOST: kbuild.html#kbuild-build-user-kbuild-build-host
-.. _KCFLAGS: kbuild.html#kcflags
-.. _prefix-map options: https://reproducible-builds.org/docs/build-path/
 .. _Reproducible Builds project: https://reproducible-builds.org/
 .. _SOURCE_DATE_EPOCH: https://reproducible-builds.org/docs/source-date-epoch/

Makefile

···
 
 # change __FILE__ to the relative path to the source directory
 ifdef building_out_of_srctree
-KBUILD_CPPFLAGS += $(call cc-option,-fmacro-prefix-map=$(srcroot)/=)
+KBUILD_CPPFLAGS += $(call cc-option,-ffile-prefix-map=$(srcroot)/=)
 KBUILD_RUSTFLAGS += --remap-path-prefix=$(srcroot)/=
 endif