ELF: Write .eh_frame_hdr explicitly after writing .eh_frame.

This lets us remove the special case from Writer::writeSections(), and also
fixes a bug where .eh_frame_hdr isn't necessarily written in the correct
order if a linker script moves .eh_frame and .eh_frame_hdr into the same
output section.

Differential Revision: https://reviews.llvm.org/D58795

llvm-svn: 355153