[Darwin] Fix availability of exp10 for watchOS, tvOS, xROS. (#98542)
Update availability information added in 1eb7f055d9a. exp10 is available
on iOS >= 7.0 and macOS >= 10.9. On all other platforms, it is available
on any version. Also drop the x86 check, as the availability only
depends on the OS version, not the target platform.
PR: https://github.com/llvm/llvm-project/pull/98542