ARM: NEON scanline functions for bilinear scaling

ARM: NEON scanline functions for bilinear scaling

General fetch->combine->store based bilinear scanline functions.
Need further optimizations and eventually will be replaced with optimal
functions one by one.
General functions should be located in pixman-arm-neon-asm-bilinear.S and
optimal functions in pixman-arm-neon-asm.S

Following general bilinear scanline functions are implemented
    over_8888_8888
    add_8888_8888
    src_8888_8_8888
    src_8888_8_0565
    src_0565_8_x888
    src_0565_8_0565
    over_8888_8_8888
    add_8888_8_8888