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

[Gabriel Dos Reis]

Roger Sayle <roger@eyesopen.com> writes:

| This patch was originally developed as a possible solution to the
| high priority PR libstdc++/3184.  Just my luck, PR 3184 was closed
| earlier today whilst this patch was being regression tested.  It
| does however protect against using __builtin functions on targets
| that don't support them.

I think we already discussed that matter in the past.  If a target
does not support a __builtin_xxx, then it falls back to a library xxx
definition. libmath/stubs.c is the right place to put missing xxx
definitions. 

[...]

| Is this OK for mainline, even though it no longer has a PR?

No.

Thanks,

-- Gaby
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