Bug 1101070 - Add missing OSX build flag for the new replace-malloc symbol. r=njn,r=khuey

Bug 818922 added a new replace-malloc symbol that the OSX linker tricks need
to know about.
Bug 1098967 - Use native Win32 APIs to append to the LogAlloc output. r=nfroyd

Because it turns out that the POSIX API the CRT exposes doesn't do O_APPEND
in a sane manner.