Move the global safepoint to one page before static space

Move the global safepoint to one page before static space

New attempt at finding an optimal location for the global safepoint
page.  Hopefully this solution will cover all our use cases for
the foreseeable future.

  - Relative to static space for architectures with small immediates.

  - Also addresses the requirements of sb-dynamic-core, which needs
    the safepoint to have an absolute address on x86(-64).

Hence, a new macro !gencgc-space-setup -- tweaks space parameters to
squeeze in the safepoint page, while also simplifying parms.lisp for
some platforms.