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

[Gabriel Dos Reis]

Roger Sayle <roger@eyesopen.com> writes:

| This patch cleans up libstdc++-v3/include/c_shadow/bits/std_cmath.h
| following my earlier patch to define sinf and cosf in libmath/stubs.c
| if needed.  http://gcc.gnu.org/ml/gcc-patches/2001-12/msg02509.html

The patch is fine. 

Thanks!

-- Gaby
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