Log Message:
Update to sbcl 2.3.6.

Add dependency on math/mpfr.

Changes in sbcl-2.3.6 relative to sbcl-2.3.5

    * platform support:
	* restore building contribs on riscv; (#2002930)
	* shorter constant-loading sequences on riscv;
	* on OpenBSD, map the stack without executable permission
          (thanks to Sébastien Marie)
	* Restore OpenBSD/arm64 for OpenBSD 7.3 (#2024003, reported by
          Robert Palm)
    * bug fix: AREF on multidimensional arrays with the wrong number
      of indices now signals an error. (#2022327, reported by EU)
    * bug fix: the nature of NIL as both STRING and SEQUENCE is
      correctly handled in the compiler's handling of string
      functions. (#2023118, reported by Patrick Poitras)
    * bug fix: the sb-bsd-sockets tests no longer fail on systems
      configured without IPv6. (reported by Will Senn)
    * bug fix: the compiler no longer transforms into
      incompletely-known functions (#1824351)
    * optimization: better (the word-sized-type (ASH word-sized
      word-sized)) when the result can overflow. For arm64 and x86-64.
    * optimization: better unoptimized calls to CEILING, FLOOR.
    * optimization: functions that involve coercing floats now cons
      less. For arm64 and x86-64.
    * optimization: array displacement to simple arrays is slightly
      faster. (reported by Shubhamkar Ayare)