Attempt to fix #2512 and #2063; add +RTS -xm<address> -RTS option

On x86_64, the RTS needs to allocate memory in the low 2Gb of the
address space.  On Linux we can do this with MAP_32BIT, but sometimes
this doesn't work (#2512) and other OSs don't support it at all
(#2063).  So to work around this:

  - Try MAP_32BIT first, if available.

  - Otherwise, try allocating memory from a fixed address (by default
    1Gb)

  - We now provide an option to configure the address to allocate
    from.  This allows a workaround on machines where the default
    breaks, and also provides a way for people to test workarounds
    that we can incorporate in future releases.

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