basic/missing_syscall: add missing_fchmodat2()

Follow-up for 8b45281daa
and preparation for later commits.

Since libcs are more interested in the POSIX `fchmodat(3)`, they are
unlikely to provide a direct wrapper for this syscall. Thus, the headers
we examine to set `HAVE_*` are picked somewhat arbitrarily.

Also, hook up `try_fchmodat2()` in `test-seccomp.c`. (Also, correct that
function's prototype, despite the fact that mistake would not matter in
practice)

Co-authored-by: Mike Yuan <me@yhndnzj.com>

src/basic/missing_syscall.h

@@ -32,6 +32,21 @@
 
 /* ======================================================================= */
 
+#if !HAVE_FCHMODAT2
+static inline int missing_fchmodat2(int dirfd, const char *path, mode_t mode, int flags) {
+#  ifdef __NR_fchmodat2
+        return syscall(__NR_fchmodat2, dirfd, path, mode, flags);
+#  else
+        errno = ENOSYS;
+        return -1;
+#  endif
+}
+
+#  define fchmodat2 missing_fchmodat2
+#endif
+
+/* ======================================================================= */
+
 #if !HAVE_PIVOT_ROOT
 static inline int missing_pivot_root(const char *new_root, const char *put_old) {
         return syscall(__NR_pivot_root, new_root, put_old);