core: long double issue with Mac M1 chip (ARM)

Reported first by Andy Tu here: https://groups.google.com/g/ns-3-users/c/TlurOJilcbg/m/f5qf31rbBAAJ

Newest Macs use the ARM M1 chip, which has a different size for long double.

Intel chips use 80 bits for long double, while ARM architectures use 64 bits. Unfortunately this has been confirmed here: https://developer.apple.com/forums/thread/673482