Fix crash in patchELFPHDRTable when no functions are modified.

Summary:
patchELFPHDRTable was asserting that it could not find an entry
for .eh_frame_hdr in SectionMapInfo when no functions were modified
by BOLT.

This just changes code to skip modifying GNU_EH_FRAME program headers
hen SectionMapInfo is empty.  The existing header is copied and written
instead.

(cherry picked from FBD3557481)