Fix GHCi segfault during startup on linux-powerpc (#2972).

Slightly modified version of a patch from Ben Collins <bcollins@ubuntu.com>
who did the final debugging that showed the segfault was being caused the
memory protection mechanism.

Due to the requirement of "jump islands" to handle 24 bit relative jump
offsets, GHCi on PowerPC did not use mmap to load object files like the
other architectures. Instead, it allocated memory using malloc and fread
to load the object code. However there is a quirk in the GNU libc malloc
implementation. For memory regions over certain size (dynamic and
configurable), malloc will use mmap to obtain the required memory instead
of sbrk and malloc's call to mmap sets the memory readable and writable,
but not executable. That means when GHCi loads code into a memory region
that was mmapped instead of malloc-ed and tries to execute it we get a
segfault.

This solution drops the malloc/fread object loading in favour of using
mmap and then puts the jump island for each object code module at the
end of the mmaped region for that object.

This patch may also be a solution on other ELF based powerpc systems
but does not work on darwin-powerpc.

MERGED from commit 3e6c9308