[ELF] Prevent crash in writing an .ARM.exidx sentinel entry.

We might crash in 'ARMExidxSentinelSection::writeTo()' because it expected
the sentinel entry to be put in the same 'InputSectionDescription' as
the last real entry. This assumption fails if the last output section command
for .ARM.exidx is anything but an input section description, because in this
case 'OutputSection::addSection()' creates a new 'InputSectionDescription'.

Differential Revision: https://reviews.llvm.org/D41105

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@320668 91177308-0d34-0410-b5e6-96231b3b80d8