This commit fixes errors occurring during Android export template
builds.
This required modification in third-party library (libpng) to compile
with NEON. Most likely a similar patch will be applied by them and we
could then replace the modified version.
Uses the new structure agreed upon in #6157, but the thirdparty/ folder
does not behave following a logic similar to that of modules/ yet.
The png driver can't be moved to a module as discussed in #6157, as it's
required by core together with a few other ImageLoader implementations
(see drivers/register_driver_types.cpp:register_core_driver_types())
Dropped the possibility to disable PNG support, it's a core component
of Godot.
