Extract the log2 code out of e_pow.c to implement log2 function.

 * log2.c:
   * New file containing the parts of e_pow.c that implemented a log2
     function for use in pow().
 * GNUmakefile:
   * Compile log2.c as part of the build.
Use new log2 function in C

 * code/exports.lisp:
   * Export %LOG2.
 * code/irrat.lisp:
   * Define %log2
   * Use %log2 instead of log2. (This needs work)
 * compiler/float-tran.lisp:
   * Use %log2 instead of log2 in the deftransforms.