kbuild: remove ARCH_{CPP,A,C}FLAGS

These flags were added by commit 61754c18 ("kbuild: Allow arch
Makefiles to override {cpp,ld,c}flags") to allow ARC to override -O2.

We did not see any other usage after all. Now that ARC switched to
CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3, there is no more user of
these variables.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Documentation/kbuild/makefiles.rst

@@ -959,13 +959,6 @@ When kbuild executes, the following steps are followed (roughly):
 	$(KBUILD_ARFLAGS) set by the top level Makefile to "D" (deterministic
 	mode) if this option is supported by $(AR).
 
-    ARCH_CPPFLAGS, ARCH_AFLAGS, ARCH_CFLAGS   Overrides the kbuild defaults
-
-	These variables are appended to the KBUILD_CPPFLAGS,
-	KBUILD_AFLAGS, and KBUILD_CFLAGS, respectively, after the
-	top-level Makefile has set any other flags. This provides a
-	means for an architecture to override the defaults.
-
     KBUILD_LDS
 
 	The linker script with full path. Assigned by the top-level Makefile.

Makefile

@@ -668,11 +668,6 @@ RETPOLINE_VDSO_CFLAGS := $(call cc-option,$(RETPOLINE_VDSO_CFLAGS_GCC),$(call cc
 export RETPOLINE_CFLAGS
 export RETPOLINE_VDSO_CFLAGS
 
-# The arch Makefile can set ARCH_{CPP,A,C}FLAGS to override the default
-# values of the respective KBUILD_* variables
-ARCH_CPPFLAGS :=
-ARCH_AFLAGS :=
-ARCH_CFLAGS :=
 include arch/$(SRCARCH)/Makefile
 
 ifdef need-config
@@ -927,11 +922,10 @@ include scripts/Makefile.kasan
 include scripts/Makefile.extrawarn
 include scripts/Makefile.ubsan
 
-# Add any arch overrides and user supplied CPPFLAGS, AFLAGS and CFLAGS as the
-# last assignments
-KBUILD_CPPFLAGS += $(ARCH_CPPFLAGS) $(KCPPFLAGS)
-KBUILD_AFLAGS   += $(ARCH_AFLAGS)   $(KAFLAGS)
-KBUILD_CFLAGS   += $(ARCH_CFLAGS)   $(KCFLAGS)
+# Add user supplied CPPFLAGS, AFLAGS and CFLAGS as the last assignments
+KBUILD_CPPFLAGS += $(KCPPFLAGS)
+KBUILD_AFLAGS   += $(KAFLAGS)
+KBUILD_CFLAGS   += $(KCFLAGS)
 
 KBUILD_LDFLAGS_MODULE += --build-id
 LDFLAGS_vmlinux += --build-id