fix RTLD_NEXT on x86_64

the return address was being truncated to 32 bits, preventing the
dlsym code from determining which module contains the calling code.

src/ldso/x86_64/dlsym.s

@@ -2,5 +2,5 @@
 .global dlsym
 .type dlsym,@function
 dlsym:
-	mov (%rsp),%edx
+	mov (%rsp),%rdx
 	jmp __dlsym