kbuild: Never respect CONFIG_WERROR / W=e to fixdep

The fixdep hostprog may be built multiple times during a single build.
Once during the configuration phase and later during the regular phase.
As only the regular build phase respects CONFIG_WERROR / W=e, the
compiler flags might change between the phases, leading to rebuilds.

Example, the rebuilds will happen twice on each invocation of the build:

  $ make allyesconfig prepare
  make[1]: Entering directory '/tmp/deleteme'
    HOSTCC  scripts/basic/fixdep
  #
  # No change to .config
  #
    HOSTCC  scripts/basic/fixdep
    DESCEND objtool
    INSTALL libsubcmd_headers
  make[1]: Leaving directory '/tmp/deleteme'

Fix the compilation flags used for scripts/basic/ before
scripts/Makefile.warn is evaluated to stop CONFIG_WERROR / W=e
influencing the fixdep build to avoid the spurious rebuilds.

Fixes: 7ded7d3 ("scripts/Makefile.extrawarn: Respect CONFIG_WERROR / W=e for hostprogs")
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260422-kbuild-scripts-basic-werror-v1-1-8c6912ff22e0@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>

Author: t-8ch

Makefile

@@ -655,6 +655,8 @@ export RCS_FIND_IGNORE := \( -name SCCS -o -name BitKeeper -o -name .svn -o    \
 
 # Basic helpers built in scripts/basic/
 PHONY += scripts_basic
+scripts_basic: KBUILD_HOSTCFLAGS := $(KBUILD_HOSTCFLAGS)
+scripts_basic: KBUILD_HOSTLDFLAGS := $(KBUILD_HOSTLDFLAGS)
 scripts_basic:
 	$(Q)$(MAKE) $(build)=scripts/basic