Don't emit warning on missing directories

Some directories (e.g., /product/bin, etc.) in ld.config.txt may
not exist in some devices. Since many of them are optional directories,
don't emit warning when realpath() gives ENOENT for the paths.

Test: m -j
Change-Id: Ic4fa7db05bde53d3aa5df47291e83b4cdc09aa1f