[libstdc++ patch] Check for __builtins in std_cmath.h

[David Edelsohn]

>>>>> Roger Sayle writes:

Roger> David, can you check that PR 3184 is still fixed on powerpc-ibm-aix4.3
Roger> with both of these patches applied?  One advantage of this fix to that
Roger> problem is that g++ should now generate machine instructions where
Roger> available when using c_shadow.  For example, sqrt(float) on solaris 2.7
Roger> will generate the fsqrts instruction and only call the libmath stub if
Roger> necessary to set errno.

	I rebuilt libstdc++-v3 with the two patches and re-ran make check.
There were no new failures.  As long as v3 provides the float functions
when not present on the native target, everything should be okay.

Cheers, David