Update gpu.go initGPUHandles() to declare gpuHandles variable before reading it. This resolves an "invalid memory address or nil pointer dereference" error. Update dyn_ext_server.c to avoid setting the RTLD_DEEPBIND flag under __TERMUX__ (Android).
This also refines the build process for the ext_server build.
This switches darwin to dynamic loading, and refactors the code now that no static linking of the library is used on any platform