[Patch, Fortran] Use __builtin_expect in libgfortran

[Tobias Burnus]

This patch replaces

  if (unlikely fulfilled condition)
    runtime_error (...);

by
  if (__builtin_expect (unlikely fulfilled condition, 0))
    runtime_error (...);

to speed up the library.

See: http://www.imodulo.com/gnu/gcc/Other-Builtins.html

"The use of __builtin_expect has now two uses:

 - on platforms where conditional jump instructions have prediction bits
   this information can be used to set them

 - on all architectures it should now be possible to use
	-freorder-blocks
   to use the information to put the basic blocks in an order which
   guarantees an optimal straight pass through the code for the
   normal case."
(quote from http://sources.redhat.com/ml/libc-hacker/2000-04/msg00129.html
 -freorder-blocks is enabled with -O2)


I hope that I have not missed any place or wrongly marked something as
likely/unlikely which is not.


Build and check-gfortraned on x86-64-linux
OK for the trunk?

Tobias
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