Adding IFTODT for more compatibility

[kateinoigakukun]

Currently IFTODT is not defined in wasi-libc, even though DT_XXX family are defined.

wasi-libc/libc-top-half/musl/include/dirent.h

Lines 50 to 65 in ad51334

	#if defined(_GNU_SOURCE) || defined(_BSD_SOURCE)
	#ifdef __wasilibc_unmodified_upstream /* Use alternate WASI libc headers */
	#define DT_UNKNOWN 0
	#define DT_FIFO 1
	#define DT_CHR 2
	#define DT_DIR 4
	#define DT_BLK 6
	#define DT_REG 8
	#define DT_LNK 10
	#define DT_SOCK 12
	#define DT_WHT 14
	#define IFTODT(x) ((x)>>12 & 017)
	#define DTTOIF(x) ((x)<<12)
	#endif
	int getdents(int, struct dirent *, size_t);
	#endif

On the ruby side, we faced a compatibility issue due to missing IFTODT ruby/ruby#5614 because we assume IFTODT is always defined when DT_UNKNOWN is defined, and other platforms are compatible with this.

Do you think it would be worth adding IFTODT?

[sunfishcode]

Yes, it looks like we should add a IFTODT.