manual: Update @standards for [__]va_copy.

[Andreas Schwab]

On Jun 17 2017, Zack Weinberg <zackw@panix.com> wrote:

> Delete the clause beginning "before GCC 3.0", as I believe compilers
> that old will no longer work with glibc's headers anyway.

We still support ancient GCC versions.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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