x86_64: Don't assume 8-byte pointer size

x86_64: Don't assume 8-byte pointer size

Since pointer in x32 is 4 bytes, add x86_64-support.pl to define
pointer_size and pointer_register based on flavour to support
stuctures like:

struct {  void *ptr; int blocks;  }

This fixes 90-test_sslapi.t on x32.  Verified with

$ ./Configure shared linux-x86_64
$ make
$ make test

and

$ ./Configure shared linux-x32
$ make
$ make test

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/10988)