Automake's dependency tracking seems to be confused by our asm
generation double-indirection. It's sufficient to have just
CLEANFILES = $(CELT_AM_SOURCES_ARM_ASM:.s.in=-gnu.S)
to fix the error about 'armopts-gnu.S' not being cleaned, but
celt_pitch_xcorr_arm-gnu.S is also not cleaned without raising
a warning. Therefore we explicitly clean both sets of ARM_ASM.