build: Strip spurious labels

The implementation of 25cb0c1a involves lots of spurious labels.

The effect of keeping those labels around is making debugging harder.
Those labels are meaningless, and complicate the disassembly. Also,
gdb can't tell the difference between them and function entry points.

This new strip command is irrelevant to any usage of Libav that would
have used the old fully stripped version, because the old one was for
non-debug use.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>