Remove -fexceptions configure test

[Roland McGrath]

It sure would be nice if places using special compiler flags for
specific files said why they are doing what they are doing.  The
uses-callbacks variable is a nice implicit documentation for the
cases it's used in.  Yet we have a few instances that use
"$(uses-callbacks) -fexceptions".  This is redundant--but not
completely obviously so, because there is no explanation for
-fexceptions there.

What is the reason for -fexceptions in all those other places?  If
each is actually a uses-callbacks case, then we should use that
variable to clarify that in the documentation.  I think that's the
actual case for all the stdio cases, because they can lead to
callbacks via _IO_JUMPS.

Do we have any -fexceptions cases that are not actually uses-callbacks?