Native library copying in BuildMicrobenchmark.gmk cause dups on macOS

`COPY_MICROBENCHMARK_NATIVE` in make/test/BuildMicrobenchmark.gmk copies a list of files with "flattening". This results in duplicate make rules, if the same file name is present in multiple source directories.

This is likely caused by JDK-8253620, which adds debug symbols on macOS. These have the same name as the lib, but in a .dSYM subdir.