process DT_RELR relocations in ldso-startup/static-pie

commit d32dadd60efb9d3b255351a3b532f8e4c3dd0db1 added DT_RELR
processing for programs and shared libraries processed by the dynamic
linker, but left them unsupported in the dynamic linker itseld and in
static pie binaries, which self-relocate via code in dlstart.c.

add the equivalent processing to this code path so that there are not
arbitrary restrictions on where the new packed relative relocation
form can be used.

ldso/dlstart.c

@@ -140,6 +140,21 @@ hidden void _dlstart_c(size_t *sp, size_t *dynv)
 		size_t *rel_addr = (void *)(base + rel[0]);
 		*rel_addr = base + rel[2];
 	}
+
+	rel = (void *)(base+dyn[DT_RELR]);
+	rel_size = dyn[DT_RELRSZ];
+	size_t *relr_addr = 0;
+	for (; rel_size; rel++, rel_size-=sizeof(size_t)) {
+		if ((rel[0]&1) == 0) {
+			relr_addr = (void *)(base + rel[0]);
+			*relr_addr++ += base;
+		} else {
+			for (size_t i=0, bitmap=rel[0]; bitmap>>=1; i++)
+				if (bitmap&1)
+					relr_addr[i] += base;
+			relr_addr += 8*sizeof(size_t)-1;
+		}
+	}
 #endif
 
 	stage2_func dls2;