Fix the ABI dump directory name for vendor libraries

The condition for vendor libraries is inconsistent between
getRefAbiDumpFile and linkSAbiDumpFiles. It causes the ABI check not to
be triggered for vendor libraries. This commit fixes linkSAbiDumpFiles.
The vendor libraries' ABI dumps are placed in the "platform" directory
for now. A follow-up commit will allow configuring the dump directory
paths.

Test: make
Bug: 227282691
Change-Id: Iad8f92145bce6264bf51efb4b8180bb917a1476d