powerpc/boot: Separate BOOTCFLAGS from BOOTASFLAGS

BOOTCFLAGS no longer contains anything that BOOTASFLAGS needs (except
-pipe). Separate them to avoid fragility with cross-contamination of
flags which has caused several build problems.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/lkml/CAHk-=whyWUdJDeOBN1hRWYSkQkvzYiQ5RbSW5rJjExgnbSNX9Q@mail.gmail.com/Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230606064657.183969-4-npiggin@gmail.com

arch/powerpc/boot/Makefile

@@ -76,7 +76,7 @@ else
 BOOTTARGETFLAGS	+= -mlittle-endian
 endif
 
-BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTTARGETFLAGS) $(BOOTCFLAGS)
+BOOTAFLAGS	:= -D__ASSEMBLY__ $(BOOTTARGETFLAGS) -pipe
 
 BOOTARFLAGS	:= -crD