Avoid empty .eh_frame sections.

Strip on OpenBSD does not correctly handle an empty .eh_frame section
and produces broken binaries in that case. Currently lld creates such
an empty .eh_frame section, despite the fact that the OpenBSD crtend.o
explicitly inserts a terminator. The Linux LSB "standard":

https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html#EHFRAME

explicitly says that

    The .eh_frame section shall contain 1 or more Call Frame Information (CFI) records.

This diff includes a test that specifically tests the issue I'm seeing
on OpenBSD.

Patch by Mark Kettenis!

llvm-svn: 301931