mpg123: bump to 1.20.0, providing better optimizations for ARM and AArch64

As suggested by Gustavo, this patch bumps mpg123 to 1.20.0, and
therefore superseds the patch initially provided by Sven Neumann. In
1.20.0, there is a new CPU type arm_fpu, which will compile code for
both VFP and NEON, and choose at runtime which one to use.

Similarly for AArch64, the option --with-cpu=aarch64 will compile both
the NEON and generic decoders, and select at runtime which one to use.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>