Add a target_info definition for NetBSD. The definition is based
on the one used by FreeBSD, with libcxxrt replaced by libc++abi,
and using llvm-libunwind since we need to use its unwinder
implementation to build anyway.
Additionally, XFAIL the 30 tests that fail because of non-implemented
locale features. According to the manual, NetBSD implements only
LC_CTYPE part of locale handling. However, there is a locale database
in the system and locale specifications are validated against it,
so it makes sense to list the common locales as supported.
If I'm counting correctly, this change enables additional 43 passing