Write .eh_frame and .eh_frame_hdr after reordering BBs

Summary:
This patch adds logic to detect when the binary has extra space
reserved for us via the __flo_storage symbol. If this symbol is present,
it means we have extra space in the binary to write extraneous information.
When we write a new .eh_frame, we cannot discard the old .eh_frame because
it may still contain relevant information for functions we do not reorder.
Thus, we write the new .eh_frame into __flo_storage and patch the current
.eh_frame_hdr to point to the new .eh_frame only for the functions we touched,
generating a binary that works with a bi-.eh_frame model.

(cherry picked from FBD2639326)