Fix make clean for asm offset files

Automatically created assembly offset files added to CLEAN-OBJS list
for proper cleanup. This will fix following build error:
1) Build for the workstation
./conigure
make
make clean
2) Build for ARM platform
./configure --target=armv7-linux-gcc
make ==> this will fail because it uses old asm_*_offset.asm files

Change-Id: Id5275c470390ca81b8db086a15ad75af39b80703

libs.mk

diff --git a/libs.mk b/libs.mk
index 1819b24..e3b2e2c 100644
--- a/libs.mk
+++ b/libs.mk
@@ -262,14 +262,17 @@
     asm_com_offsets.asm: $(VP8_PREFIX)common/asm_com_offsets.c.S
 	grep EQU $< | tr -d '$$\#' $(ADS2GAS) > $@
     $(VP8_PREFIX)common/asm_com_offsets.c.S: vp8/common/asm_com_offsets.c
+    CLEAN-OBJS += asm_com_offsets.asm $(VP8_PREFIX)common/asm_com_offsets.c.S
 
     asm_enc_offsets.asm: $(VP8_PREFIX)encoder/asm_enc_offsets.c.S
 	grep EQU $< | tr -d '$$\#' $(ADS2GAS) > $@
     $(VP8_PREFIX)encoder/asm_enc_offsets.c.S: vp8/encoder/asm_enc_offsets.c
+    CLEAN-OBJS += asm_enc_offsets.asm $(VP8_PREFIX)encoder/asm_enc_offsets.c.S
 
     asm_dec_offsets.asm: $(VP8_PREFIX)decoder/asm_dec_offsets.c.S
 	grep EQU $< | tr -d '$$\#' $(ADS2GAS) > $@
     $(VP8_PREFIX)decoder/asm_dec_offsets.c.S: vp8/decoder/asm_dec_offsets.c
+    CLEAN-OBJS += asm_dec_offsets.asm $(VP8_PREFIX)decoder/asm_dec_offsets.c.S
 else
   ifeq ($(filter rvct,$(TGT_CC)), $(TGT_CC))
     asm_com_offsets.asm: obj_int_extract