[PATCH] [libc++] Support Newlib as libc++'s C library (on ARM) [cmath part, part 1]

[JF Bastien]

================
Comment at: include/support/newlib/math.h:17
@@ +16,3 @@
+
+#if defined(__arm__) && !defined(__LP64__)
+double nexttoward(double d, long double td);
----------------
jfb wrote:
> Why `__arm__`? It seems better to key off `long double` being the same as `double`. I'm not sure if clang has a macro for this, but it should.
Discussed on IRC:
```
#if __SIZEOF_DOUBLE__ == __SIZEOF_LONG_DOUBLE__
```

http://reviews.llvm.org/D5419