Build ld.config.txt for older VNDK versions

Until now, the ld.config.$VER.txt for $VER older than
PLATFORM_VNDK_VERSION was installed from a prebuilt stored under
/prebuilts/vndk/v$VER. At runtime, the VNDK version needed by the
vendor partition (ro.vndk.version) determines which
ld.config.$VER.txt is loaded and this configuration is used for
both system and vendor processes. In a system-only-upgrade
scenario, where the system partition is a newer Android version
than the vendor partition, this is a problem because the prebuilt
linker config file will not reflect the most recent
system partition related changes made in the newer Android
version.

To fix this problem, this change builds ld.config.$VER.txt for
older VNDK versions from the source tree by getting the lists of
VNDK libraries from the /prebuilts/vndk/v{VER} directory.

Test: m -j ld.config.txt ld.config.28.txt ld.config.vndk_lite.txt
Bug: 74658756
Change-Id: I1fa2469fc0bce2f2aab507104cd9717b6112d738