fix dl_iterate_phdr in static PIE binaries

analogous to commit 5bf7eba213, use of
AT_PHDR/PT_PHDR does not actually work to find the program base, and
the method with _DYNAMIC vs PT_DYNAMIC must be used as an alternative.

patch by Shiz, along with testing to confirm that this fixes unwinding
in static PIE.

src/ldso/dl_iterate_phdr.c

@@ -4,6 +4,9 @@
 
 #define AUX_CNT 38
 
+__attribute__((__weak__, __visibility__("hidden")))
+extern const size_t _DYNAMIC[];
+
 static int static_dl_iterate_phdr(int(*callback)(struct dl_phdr_info *info, size_t size, void *data), void *data)
 {
 	unsigned char *p;
@@ -20,6 +23,8 @@ static int static_dl_iterate_phdr(int(*callback)(struct dl_phdr_info *info, size
 		phdr = (void *)p;
 		if (phdr->p_type == PT_PHDR)
 			base = aux[AT_PHDR] - phdr->p_vaddr;
+		if (phdr->p_type == PT_DYNAMIC && _DYNAMIC)
+			base = (size_t)_DYNAMIC - phdr->p_vaddr;
 		if (phdr->p_type == PT_TLS)
 			tls_phdr = phdr;
 	}