drop direct use of stat syscalls in internal __map_file

this function is used to implement some baseline ISO C interfaces, so
it cannot call any of the stat functions by their public names. use
the namespace-safe __fstatat instead.

src/time/__map_file.c

@@ -1,16 +1,16 @@
+#define _BSD_SOURCE
 #include <sys/mman.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 #include "syscall.h"
-#include "kstat.h"
 
 const char unsigned *__map_file(const char *pathname, size_t *size)
 {
-	struct kstat st;
+	struct stat st;
 	const unsigned char *map = MAP_FAILED;
 	int fd = sys_open(pathname, O_RDONLY|O_CLOEXEC|O_NONBLOCK);
 	if (fd < 0) return 0;
-	if (!syscall(SYS_fstat, fd, &st)) {
+	if (!__fstatat(fd, "", &st, AT_EMPTY_PATH)) {
 		map = __mmap(0, st.st_size, PROT_READ, MAP_SHARED, fd, 0);
 		*size = st.st_size;
 	}