dir.${section}: downgrade "can't resolve" diag

When parsing a dir.${section} line in ld.config.txt, the linker uses
realpath() to resolve each directory path. If realpath() fails, the linker
uses the non-resolved path instead. Previously, it issued a warning to
stderr and logcat, but realpath() is expected to fail on /postinstall when
running a binary in /data/local/tmp, so downgrade the warning to an INFO()
notice.

Bug: http://b/120996057
Test: copy ping to /data/local/tmp, run it, verify no errors
Test: bionic unit tests
Change-Id: I2104084e12ea18840230997408559299bf3bddac