recognize 7000, 7100, 7200, and 8x00.
<li> HPPA: gcc 3.2 introduces a <code>-mschedule=7200</code> etc parameter,
     which could be driven by an exact hppa cpu type.
<li> Mips: config.guess should say mipsr3000, mipsr4000, mipsr10000, etc.
     "hinv -c processor" gives lots of information on Irix.  Standard
     config.guess appends "el" to indicate endianness, but
     <code>AC_C_BIGENDIAN</code> seems the best way to handle that for GMP.
<li> PowerPC: The function descriptor nonsense for AIX is current