[InferAttrs] Add writeonly to all the math functions

All of these functions would be `readnone`, but can't be on platforms
where they can set `errno`. A `writeonly` function with no pointer
arguments can only write (but never read) global state.

Writeonly theoretically allows these calls to be CSE'd (a writeonly call
with the same arguments will always result in the same global stores) or
hoisted out of loops, but that's not implemented currently.

There are a few functions in this list that could be `readnone` instead
of `writeonly`, if someone is interested.

Differential Revision: https://reviews.llvm.org/D116426