Disable some overloaded math functions for libc++ 3.8.0 and higher, and thus
unbreak the build.

This is because this port defines its own versions of a number of <cmath>
functions, which cause the ambiguity. These functions are only declared in
<cmath> for libc++ 3.8.0 and later, so fix it by disabling the custom
functions for those versions of libc++.

PR: 209652
Submitted by: dim