[patch] port-irix.c: refine jlimit support

[Ayamura KIKUCHI]

> You sure this is complete?  Last time someone proposed a patch like this
> it was incomplete.  The patch from SGI was much more involved.

I hope this is complete :-)
I can compile the modified port-irix.c with MIPSpro Compilers and use it.

/*
 *************************************************************************
 *      _MIPS_SYMBOL_PRESENT(_x)
 *
 *      Returns 1 if the symbol '_x' is defined in the current
 *      runtime environment.  Returns 0 otherwise.
 *
 *      NOTE: This is a valid check for any and all symbol types.
 *************************************************************************
 */

-- ayamura