Stop using mremap() to allocate space for trampolines
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.