Use __BIGGEST_ALIGNMENT__ instead of max_align_t (#375)

Use __BIGGEST_ALIGNMENT__ instead of max_align_t (#375)

Also, remove no longer necessary __need_STDDEF_H_misc stuff.

References:
https://github.com/WebAssembly/wasi-libc/pull/335
https://github.com/WebAssembly/wasi-sdk/issues/111
https://github.com/llvm/llvm-project/blob/2e999b7dd1934a44d38c3a753460f1e5a217e9a5/clang/lib/Headers/stddef.h#L106-L113