[lldb] [Plugins/SysV-x86_64] NetBSD is also using SysV ABI

Reenable SysV x86_64 ABI usage on NetBSD that was accidentally removed
in r364216.  This fixes numerous test failures with messages similar
to the following:

  error: Can't run the expression locally: Interpreter doesn't handle
  one of the expression's opcodes

llvm-svn: 364503