5l, 6l, 8l: Add a PT_LOAD PHDR entry for the PHDR.

5l, 6l, 8l: Add a PT_LOAD PHDR entry for the PHDR.

Per the TIS ELF spec, if a PHDR entry is present in the
program header table, it must be part of the memory image of
the program. Failure to do this makes elflint complain, and
causes some tools that manipulate ELF to crash.

R=iant, rsc
CC=dave, golang-dev
https://golang.org/cl/4650067