ARM: allow building in Thumb2 mode

configure

libavcodec/arm/aac.h

libavcodec/arm/ac3dsp_arm.S

libavcodec/arm/ac3dsp_armv6.S

libavcodec/arm/ac3dsp_neon.S

libavcodec/arm/asm.S

libavcodec/arm/dcadsp_neon.S

libavcodec/arm/dsputil_arm.S

libavcodec/arm/dsputil_armv6.S

libavcodec/arm/dsputil_neon.S

libavcodec/arm/dsputil_vfp.S

libavcodec/arm/fmtconvert_neon.S

libavcodec/arm/fmtconvert_vfp.S

libavcodec/arm/h264dsp_neon.S

libavcodec/arm/h264idct_neon.S

libavcodec/arm/mathops.h

libavcodec/arm/mdct_neon.S

libavcodec/arm/mpegaudiodsp_fixed_armv6.S

libavcodec/arm/mpegvideo_armv5te_s.S

libavcodec/arm/mpegvideo_neon.S