[libc] Fix math failures for fmod on NVPTX (#213168)

Summary:
Fixes some failing tests. Fmod's builtin has improper handling in the
NVPTX instruction so we should just use the generic version for now. The
Float128 implementation seems to have had some undefined behavior for
out of range behavior that only showed up now. Hopefully someone can
tell me if this is fixed correctly.
GitOrigin-RevId: b232d110b082c597ac518d37624d16aac3f71ab5