head broken on openbsd

[Don Stewart]

The change to use remap in Linker.c has broken the mmap Linker.c
implementation on OpenBSD, and other BSDs.

    #ifndef linux_HOST_OS /* mremap is a linux extension */
        #error ocAllocateSymbolExtras doesnt want USE_MMAP to be defined
    #endif

Do we know why this was done? USE_MMAP worked perfectly well until
recently, perhaps the remap stuff could be ifdeffed instead?

-- Don