This was causing problems because sometimes mremap() moved the memory
we had allocated from the low 2Gb to above the 2Gb boundary, causing
some linkages to fail.  There's no MAP_32BIT flag to mremap().

So now we just use mmap(MAP_ANON|MAP_32BIT) to allocated space for the
trampolines.  People without MAP_32BIT (eg. *BSD) will still have to
do something else here, such as allocating memory from a fixed
address; so I've made it slightly easier for those guys, but there's
still work to do (#2063).

One solution (that Simon PJ is advocating) is to turn on -fPIC by
default on x86-64.  This is a good solution as it removes the need for
MAP_32BIT, but doesn't work with -fvia-C, so probably this is for
later.