process-util: use clone2 on ia64

glibc does not provide clone() on ia64, only clone2. But only as a
symbol in the shared library, there's no prototype in the gblic
headers, so we have to define it, copied from the manpage.

src/basic/missing_syscall.h

@@ -654,3 +654,17 @@ static inline ssize_t missing_getdents64(int fd, void *buffer, size_t length) {
 
 #  define getdents64 missing_getdents64
 #endif
+
+/* ======================================================================= */
+
+/* glibc does not provide clone() on ia64, only clone2(). Not only that, but it also doesn't provide a
+ * prototype, only the symbol in the shared library (it provides a prototype for clone(), but not the
+ * symbol in the shared library). */
+#if defined(__ia64__)
+int __clone2(int (*fn)(void *), void *stack_base, size_t stack_size, int flags, void *arg);
+#define HAVE_CLONE 0
+#else
+/* We know that everywhere else clone() is available, so we don't bother with a meson check (that takes time
+ * at build time) and just define it. Once the kernel drops ia64 support, we can drop this too. */
+#define HAVE_CLONE 1
+#endif