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 3e6c930

rts/Linker.c

@@ -70,11 +70,12 @@
 #include <sys/wait.h>
 #endif
 
-#if !defined(powerpc_HOST_ARCH) && \
-    (   defined(linux_HOST_OS    ) || defined(freebsd_HOST_OS) || \
+#if (defined(powerpc_HOST_ARCH) && defined(linux_HOST_OS)) \
+ || (!defined(powerpc_HOST_ARCH) && \
+    (   defined(linux_HOST_OS)     || defined(freebsd_HOST_OS) || \
         defined(dragonfly_HOST_OS) || defined(netbsd_HOST_OS ) || \
         defined(openbsd_HOST_OS  ) || defined(darwin_HOST_OS ) || \
-        defined(kfreebsdgnu_HOST_OS) || defined(gnu_HOST_OS))
+        defined(kfreebsdgnu_HOST_OS) || defined(gnu_HOST_OS)))
 /* Don't use mmap on powerpc_HOST_ARCH as mmap doesn't support
  * reallocating but we need to allocate jump islands just after each
  * object images. Otherwise relative branches to jump islands can fail
@@ -90,6 +91,16 @@
 
 #endif
 
+
+/* PowerPC has relative branch instructions with only 24 bit displacements
+ * and therefore needs jump islands contiguous with each object code module.
+ */
+#if (defined(USE_MMAP) && defined(powerpc_HOST_ARCH) && defined(linux_HOST_OS))
+#define USE_CONTIGUOUS_MMAP 1
+#else
+#define USE_CONTIGUOUS_MMAP 0
+#endif
+
 #if defined(linux_HOST_OS) || defined(solaris2_HOST_OS) || defined(freebsd_HOST_OS) || defined(kfreebsdgnu_HOST_OS) || defined(dragonfly_HOST_OS) || defined(netbsd_HOST_OS) || defined(openbsd_HOST_OS) || defined(gnu_HOST_OS)
 #  define OBJFORMAT_ELF
 #  include <regex.h>    // regex is already used by dlopen() so this is OK
@@ -2797,8 +2808,26 @@ static int ocAllocateSymbolExtras( ObjectCode* oc, int count, int first )
      */
     if( m > n ) // we need to allocate more pages
     {
-        oc->symbol_extras = mmapForLinker(sizeof(SymbolExtra) * count,
+        if (USE_CONTIGUOUS_MMAP)
+        {
+            /* Keep image and symbol_extras contiguous */
+            void *new = mmapForLinker(n + (sizeof(SymbolExtra) * count),
+                                  MAP_ANONYMOUS, -1);
+            if (new)
+            {
+                memcpy(new, oc->image, oc->fileSize);
+                munmap(oc->image, n);
+                oc->image = new;
+                oc->symbol_extras = (SymbolExtra *) (oc->image + n);
+            }
+            else
+                oc->symbol_extras = NULL;
+        }
+        else
+        {
+            oc->symbol_extras = mmapForLinker(sizeof(SymbolExtra) * count,
                                           MAP_ANONYMOUS, -1);
+        }
     }
     else
     {