Fix a very stupid bug in the new aspacemgr, in which mmap "hint-style"

Fix a very stupid bug in the new aspacemgr, in which mmap "hint-style"
requests were being granted at the requested address when they should
not have been.  This was causing ppc32-linux to crash at startup
(since the wrongly-granted mapping annihilated 5 others).  This shows
the value of a multiplatform approach -- the bug applies to all
targets, yet x86 and amd64 appeared to work perfectly.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@4820