improve printf locality

[Eric Blake]

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On cygwin, where newlib is configured with --enable-newlib-io-long-double,
print consumed more than an entire page of virtual memory in stack space!
 This causes unnecessary page faults, decreased cache locality, and other
potential slowdowns.  It was because at one point in history, newlib used
to stack allocate the worst case length for %Lf output (if you have a
128-bit long double with a 15 bit exponent, this can result in more than
4900 characters).  But now newlib uses struct _reent to manage %f output,
so the storage claimed by buf[BUF] was 90% unused.

This also fixes some problems with the radix character (if newlib ever
supports non-C locales, it must use the radix in a thread-safe and
consistent manner), and optimizes %S processing to avoid malloc for short
strings.

OK to apply?

2007-05-10  Eric Blake  <ebb9@byu.net>

	* libc/stdio/vfprintf.c (_VFPRINTF_R): Fix use of decimal point
	in %f and %e.  Avoid malloc when possible for %S.
	(BUF): Improve stack locality by using smaller size.
	(MAXEXP): Define.
	(exponent): Use for smaller stack size.

- --
Don't work too hard, make some time for fun as well!

Eric Blake             ebb9@byu.net
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