[Patch] Fix libstdc++/11722

[Paolo Carlini]

Hi,

the below fixes for mainline (*) this long standing performance PR. The
final code seems sufficiently clear to me and the improvement satisfying.
For the C++ case going in the performance testsuite (P4-2400, -O2)

mainline
--------
22.190u 25.310s 0:47.68  99.6%    0+0k 0+0io 200pf+0w

mainline + patch
----------------
 0.250u  0.250s 0:00.50 100.0%    0+0k 0+0io 199pf+0w

Also, notice that, similarly to what went in for sputn, every read with
__n > __buflen (and noconv) takes advantage, *not* only the unbuffered
ones.

Tested x86-linux, will wait a few hours, in case of comments.

Paolo.

(*) After a period of testing maybe suited for 3_4-branch too.

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