Use base CClockId as ClockId type

We used to define ClockId as hsc2hs-detected ${type clockid_t} type.
Unfortunately, certain libcs (e.g. wasi-libc) define clockid_t as a
pointer type, and hsc2hs only supports detecting C
integral/floating-point types, so clockid_t will mistakenly be
detected as a floating-point type, resulting in incorrect code
generation.

We should really just use the base CClockId type here. base handles C
pointer types correctly, since it uses its own custom autoconf logic
instead of hsc2hs, and similar issues have been reported & fixed
before, see ghc/ghc!6896
and ghc/ghc!6912.

Also make tzset conditional.