Handle NetBSD specific path in findDebugBinary()

Summary:
The NetBSD Operating System installs debuginfo
files into /usr/libdata/debug, rather than other path
like in some other popular distribution.

This change makes llvm-symbolizer functional with
the basesystem executables.

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: JDevlieghere, llvm-commits

Differential Revision: https://reviews.llvm.org/D48525

llvm-svn: 335511