Fix fp16 with asan
The IFUNC resolvers that we are using here get run early, before asan had a chance to set up its plumbing, and therefore things go badly if they are compiled with asan. Turning it off makes things work again.
The gcc bug tracking this problem: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110442
Thanks to Jakub Jelinek and Florian Weimer for analyzing this and recommending the workaround.