[libc] Add __FE_DENORM to the fenv macros (#91353)

Summary:
Some targets support denormals as floating point exceptions. This is
provided as an extension in the GNU headers as __FE_DENORM.

This provides it in our headers, however I'm unsure if we should make it
internal or external. I do not think it should be in all exception as it
doesn't represent an exceptional behavior as far as the standard is
concerned, but I'm not an expert.