[Patch] Streamline __mt_alloc::_S_initialize

[Paolo Carlini]

Hi,

In _S_initialize we had 4 #ifdef __GTHREADS if(__gthread_active_p()),
in particular 2 inside loops. With the below we have only one, and the
structure of the second half of the function becomes consistent with
that of the other members.

Tested x86/x86_64-linux, check/check-performance.

Will commit later today.

Paolo.

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