Apply change in ETC file names to versioned names

ETC modules ld.config.txt, llndk.libraries.txt, and
vndksp.libraries.txt are now versioned as such:
*.$PLATFORM_VNDK_VERSION.txt

Change the logic for autogenerating Android.mk build rules
for ETC files to reflect this change in file names.

Bug: 70918357
Test: python gen_buildfiles.py
Change-Id: Ia1d062a1be7fd47a78ad338c252e556d27f308b3