Use SI_KERNEL on platforms defining it

Summary:
Linux and FreeBSD occasionally send SI_KERNEL codes, nonexistent on other platforms.

Problem caught on NetBSD.

Reviewers: joerg, sas

Subscribers: sas, lldb-commits, emaste

Differential Revision: http://reviews.llvm.org/D12659

Change by Kamil Rytarowski <n54@gmx.com>

llvm-svn: 247579