Fix 'make distclean'.

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.

Makefile.am

@@ -38,6 +38,9 @@ BUILT_SOURCES = $(CELT_SOURCES_ARM_ASM:.s=-gnu.S) \
 endif
 endif
 
+CLEANFILES = $(CELT_SOURCES_ARM_ASM:.s=-gnu.S) \
+ $(CELT_AM_SOURCES_ARM_ASM:.s.in=-gnu.S)
+
 include celt_headers.mk
 include silk_headers.mk
 include opus_headers.mk