volatile qualifier hurts single-threaded optimized case

[Richard Guenther]

bits/atomicity.h has volatile qualifiers on the _Atomic_word* arguments to
the __*_single and __*_dispatch variants of the atomic operations.  This
huts especially the single-threaded optimization variants which are usually
inlined.  Removing those qualifiers allows to reduce code size significantly
as can be seen in the following simple testcase

#include <string>
#include <cstring>

char *foo (const char *s, const char *s2)
{
 std::string ss(s);
 std::string ss2(s2);
 return strdup((ss + ss2).c_str());
}

which shrinks from 408 bytes to 388 bytes text size on x86.  Changes
are CSE in the fast path which changes from

<L14>:;
 __mem = (volatile _Atomic_word *) &this->D.15413._M_refcount;
 if (__gthrw_pthread_cancel != 0B) goto <L17>; else goto <L18>;

<L17>:;
 __result = __exchange_and_add (__mem, -1);
 goto <bb 12> (<L19>);

<L18>:;
 __result = *__mem;
 D.16636 = *__mem;
 D.16637 = D.16636 + -1;
 *__mem = D.16637;

<L19>:;
 if (__result <= 0) goto <L15>; else goto <L30>;

to

<L14>:;
 if (__gthrw_pthread_cancel != 0B) goto <L17>; else goto <L18>;

<L17>:;
 __result = __exchange_and_add (&this->D.15413._M_refcount, -1);
 goto <bb 12> (<L19>);

<L18>:;
 __result = this->D.15413._M_refcount;
 this->D.15413._M_refcount = __result + -1;

<L19>:;
 if (__result <= 0) goto <L15>; else goto <L30>;

as the _M_refcount member of basic_string is not volatile.

FYI,
Richard.