Support GNU_RELRO for static executables.

In 9ec0f03a, we added dynamic
linker support for GNU_RELRO protections. These protections
make certain regions of memory read-only, helping protect certain
data structures from accidental or deliberate modifications.

This change adds GNU_RELRO support to STATIC executables.  We can
determine if we're compiled with relro protections by examining
our own program headers, which is passed to us by the kernel
in the AT_PHDR and AT_PHNUM auxiliary vectors.

Parts of this code were stolen from the dynamic linker.

Change-Id: Ic17eb5f932218538ec25347ece314d4dc7549de1