anv: anv_entrypoints_gen.py: directly write files instead of piping

anv: anv_entrypoints_gen.py: directly write files instead of piping

This changes the output to be written as a file rather than being piped.
This had one critical advantage, it encapsulates the encoding. This
prevents bugs where a symbol (generally unicode like © [copyright]) is
printed and the system being built on doesn't have a unicode locale.

v2: - Update Android.mk
v3: - Don't generate both files at once
    - Fix Android.mk
    - drop --outdir, since the filename is passed in as an argument

Signed-off-by: Dylan Baker <dylanx.c.baker@intel.com>