elf-core: Add helper function for parsing FreeBSD NT_PRSTATUS

This avoids the use of the ELFPrStatus class, which returns to being
used only for Linux.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@186976 91177308-0d34-0410-b5e6-96231b3b80d8