Import e_log10.c from netlib as is.
Update to use a union.
Use fdlibm_setexception to signal appropriate exceptions.
Add fdlibm's log10.

 * src/code/irrat.lisp:
   * Define %log10 to use fdlibm's log10
 * src/lisp/GNUmakefile
   * Compile e_log10
 * tests/trig.lisp
   * Add tests for %log10.