sys/cdefs.h: add noexcept and noexcept_if

These macros provide the C++11 noexcept and noexcept(...) keywords if
we're compiling in a C++11 environment. Otherwise, they expand to an
empty string.

This will be used to add the required noexcept specifier to several libc
functions as required in C++11.

MFC after: 2 weeks

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1085