ld.config.txt: Link vendor vndk ns to default ns

This commit links the vndk namespace of vendor processes to the default
namespace of vendor processes.  allow_all_shared_libs is specified so
that VNDK extensions can link with other vendor libraries in
/vendor/lib[64].

Bug: 69824336

Test: Create a vndk-ext lib that links to vendor lib and use that
vndk-ext lib in a vendor executable.

Change-Id: Ibd3dc2e5f0f2e6ff9fc9677c6bcfcf87a1a70e50