ARMv6: Lay the groundwork for later patches in the series

ARMv6: Lay the groundwork for later patches in the series

Move the entire contents of pixman-arm-simd-asm.S to a new file;
ultimately this will only retain the scaled operations, so it is
named pixman-arm-simd-asm-scaled.S. Added new header file
pixman-arm-simd-asm.h, containing the macros which are the basis of
all the new ARMv6 implementations, although at this point in the
series, nothing uses them and the library should be binary-identical.