powerpc/boot: Disable power10 features after BOOTAFLAGS assignment

When building the boot wrapper assembly files with clang after
commit 648a178 ("powerpc/boot: Fix boot wrapper code generation
with CONFIG_POWER10_CPU"), the following warnings appear for each file
built:

  '-prefixed' is not a recognized feature for this target (ignoring feature)
  '-pcrel' is not a recognized feature for this target (ignoring feature)

While it is questionable whether or not LLVM should be emitting a
warning when passed negative versions of code generation flags when
building assembly files (since it does not emit a warning for the
altivec and vsx flags), it is easy enough to work around this by just
moving the disabled flags to BOOTCFLAGS after the assignment of
BOOTAFLAGS, so that they are not added when building assembly files.
Do so to silence the warnings.

Fixes: 648a178 ("powerpc/boot: Fix boot wrapper code generation with CONFIG_POWER10_CPU")
Link: ClangBuiltLinux#1839
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230427-remove-power10-args-from-boot-aflags-clang-v1-1-9107f7c943bc@kernel.org

Author: nathanchance

arch/powerpc/boot/Makefile

@@ -34,8 +34,6 @@ endif
 
 BOOTCFLAGS    := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
 		 -fno-strict-aliasing -O2 -msoft-float -mno-altivec -mno-vsx \
-		 $(call cc-option,-mno-prefixed) $(call cc-option,-mno-pcrel) \
-		 $(call cc-option,-mno-mma) \
 		 $(call cc-option,-mno-spe) $(call cc-option,-mspe=no) \
 		 -pipe -fomit-frame-pointer -fno-builtin -fPIC -nostdinc \
 		 $(LINUXINCLUDE)
@@ -71,6 +69,10 @@ BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTCFLAGS) -nostdinc
 
 BOOTARFLAGS	:= -crD
 
+BOOTCFLAGS	+= $(call cc-option,-mno-prefixed) \
+		   $(call cc-option,-mno-pcrel) \
+		   $(call cc-option,-mno-mma)
+
 ifdef CONFIG_CC_IS_CLANG
 BOOTCFLAGS += $(CLANG_FLAGS)
 BOOTAFLAGS += $(CLANG_FLAGS)