Fix fp16 with asan

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.