Fix wasm32-wasi build support

[TerrorJack]

CLOCK_REALTIME is not always a literal integer. One counter-example is wasi-libc, where it aliases an internal symbol and therefore is a pointer type. This makes it incompatible with hsc2hs #const, therefore we better use capi import here, which is guaranteed to work uniformly and fixes wasm32-wasi support.

[AshleyYakeley]

GHC doesn't support WebAssembly, does it? Is this a new effort for GHC, or some other Haskell compiler? I'd like to be able to test this if possible.

[TerrorJack]

It's a part of the work in adding WebAssembly support to GHC: https://gitlab.haskell.org/ghc/ghc/-/merge_requests/7632

There aren't instructions to build that branch yet, but the change included here can be validated on CI for normal architecture.

[AshleyYakeley]

Could you point me to the headers that define clockid_t and CLOCK_REALTIME in wasi-libc? I'd like to have a better idea of what's going on.

[TerrorJack]

Of course:

• struct __clockid is defined at https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/cloudlibc/src/common/clock.h, with comment that explains why it's an object instead of an integer literal like other POSIX platforms
• clockid_t is defined as a pointer to struct __clockid at https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/headers/public/__typedef_clockid_t.h. It's the public interface for wasi-libc users.
• CLOCK_REALTIME points to a global struct _CLOCK_REALTIME at https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/headers/public/__header_time.h
• The _CLOCK_REALTIME struct is defined at https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/cloudlibc/src/libc/time/CLOCK_REALTIME.c.
• The __WASI_CLOCKID_REALTIME magic number comes from https://github.com/WebAssembly/wasi-libc/blob/main/libc-bottom-half/headers/public/wasi/api.h#L79, which is automatically derived from the WASI spec file

Overall, clockid_t can't be an integer literal that can be detected as hsc2hs #const, at least in the case of wasm32-wasi.

[AshleyYakeley]

Merged, thanks.