[libc++] Only forward-declare ABI-functions in exception_ptr.h if they are...

[libc++] Only forward-declare ABI-functions in exception_ptr.h if they are meant to be used (#84707)

This patch fixes the unconditional forward-declarations of ABI-functions
in exception_ptr.h, and makes it dependent on the availability macro, as
it should've been from the beginning.

The declarations being unconditional break the build with libcxxrt
before 045c52ce8 [1], now they are opt-out.

[1]: https://github.com/libcxxrt/libcxxrt/commit/045c52ce821388f4ae4d119fe4fb75f1eb547b85