Use mmap to read an initial ELF header of library

Also, fixes SIGSEV during calculation of si->phdr in load_library(),
which is caused by accessing PHDR area, which is not guaranteed to be
loaded.

Note that this usually does not happen because program header area is
covered by immediately following PT_LOAD entry.  But it does not hold
always.

Bug: 6561258
Change-Id: Ie376253c773b5420279ca89044e81b1aad5a5736