[llvm] [APFloat] Add exp functions for single and double using exp/expf implementations from LLVM libc. (PR #190667)

[via llvm-commits]

dyung wrote:

@lntue, your change is still failing to build on several build bots even after your recent fix in #197328. Can you take a look and revert if you need time to fix it so that we can get the bots green again?

https://lab.llvm.org/buildbot/#/builders/174/builds/35753
https://lab.llvm.org/buildbot/#/builders/202/builds/6733
https://lab.llvm.org/buildbot/#/builders/181/builds/43300

https://github.com/llvm/llvm-project/pull/190667