arm: move all .macro/.endm to column 0
arm: do not export every asm function

Based on Libav's libavutil/arm/asm.S. Also prevents having the same
label twice for every function on systems not defining EXTERN_ASM.
Clang's integrated assembler does not like it.
arm: correct x264_mc_chroma_neon function declaration
arm: use the weight_fn_t typedef for mc weight function arrays