debloat: use __syscall instead of syscall where possible

don't waste time (and significant code size due to function call
overhead!) setting errno when the result of a syscall does not matter
or when it can't fail.

src/stdio/fopen.c

@@ -27,7 +27,7 @@ FILE *fopen(const char *filename, const char *mode)
 	f = __fdopen(fd, mode);
 	if (f) return f;
 
-	syscall(SYS_close, fd);
+	__syscall(SYS_close, fd);
 	return 0;
 }