Fixes to format preprocessing for Clang on Windows

[HJfod]

Clang on Windows defines _MSC_VER, but it doesn't seem to understand _Printf_format_string_, leading to errors code using format string parameters as it fails to understand that the parameters are format strings. However, the __attribute__((format)) syntax seems to work just fine, and so this PR enables that for Clang on Windows.

For context, I am using clang (19.1.0) instead of clang-cl (because the latter doesn't work for some unknown reasons). I've checked that this still works on MSVC, though I don't know about the rest of the platforms because I didn't realize GitHub wouldn't run CI when I made the commit. Oh well, I'll see when CI runs for the PR, and push new commits to fix it if something goes wrong.

[saghul]

Do you know how we could add a Clang on windows CI target?

[HJfod]

I believe just copying the action that uses clang-cl and making it use clang instead should work? Not entirely sure, can test it out on my fork

[saghul]

Please add it to this PR so we test it there too going forward.

[saghul]

Can you make this change also in quickjs.h, for consistency?

[saghul]

Can you make this change also in quickjs.h, for consistency?

[saghul]

I thought clang also defined GNUC, doesn't it?

[HJfod]

At least locally that seems to not be the case when compiling on Windows, though not sure why

[HJfod]

I keep running into quirks with weird things not being compatible with Clang (for example the u64 literal suffix, use of fopen etc. over fopen_s), and I don't have the time myself to work on fixing those, so this PR is most likely going to have be closed for becoming stale.