[Patch] Fix libstdc++/12791

[Paolo Carlini]

Hi,

it took me some time, but I find the result quite satisfying.

The new _M_extract_num is pretty general, only the initialization
of __mult is limited for the moment to __len = 1, 2, 4, the values
actually used in the library, otherwise (by just adding a pow() call
or something like that) the code is general and works ok up to the
the size of the int type.

This implies, among other things, that I didn't consider replacing
the division (__mult /= 10 -> __max *= 10, __min *= 10) neither I
have a special case for __len == 2 (the most common value).

Since we are now avoiding basic_string and atoi() the final code
is anyway pretty fast (besides not being affected by the iterator
bug, of course ;)

Please have a look: tomorrow (italian time) I mean to commit the
work (or variants of it ;)

Tested x86-linux, as usual.

Paolo.

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