Fix segfaults on SELinux machines; fixes #7629

Patch from wgmitchener.

From the ticket:
The two addresses (adjustorStub and code) contain the same memory
(double-mapped), but one is writable while the other is executable.
This is how libffi works around the SELinux restrictions. On
non-SELinux systems the code and data addresses are probably the same.

rts/Adjustor.c

@@ -389,7 +389,7 @@ createAdjustor(int cconv, StgStablePtr hptr,
         int sz = totalArgumentSize(typeString);
 
         adjustorStub->call[0] = 0xe8;
-        *(long*)&adjustorStub->call[1] = ((char*)&adjustorCode) - ((char*)adjustorStub + 5);
+        *(long*)&adjustorStub->call[1] = ((char*)&adjustorCode) - ((char*)code + 5);
         adjustorStub->hptr = hptr;
         adjustorStub->wptr = wptr;