Auto merge of rust-lang#146019 - joboet:better-dlsym, r=tgross35

std: optimize `dlsym!` macro and add a test for it

The `dlsym!` macro always ensures that the name string is nul-terminated, so there is no need to perform the check at runtime. Also, acquire loads are generally faster than a load and a barrier, so use them. This is only false in the case where the symbol is missing, but that shouldn't matter too much.

Author: bors