+RTS -xbXXXXX sets the "heap base" to 0xXXXXXX

+RTS -xbXXXXX sets the "heap base" to 0xXXXXXX
When debugging the GC and storage manager we often want repeated runs
of the program to allocate memory at the same addresses, so that we
can set watch points.  Unfortunately the OS doesn't always give us
memory at predictable addresses.  This flag gives the OS a hint as to
where we would like our memory allocated.  Previously I did this by
changing the HEAP_BASE setting in MBlock.h and recompiling, this patch
just adds a flag so I don't have to recompile.