AIX: make sure that the arflags value includes the command letter (r)

In previous OpenSSL versions, this letter was part of the make
variable AR.  However, following the usual convention (read: GNU),
this letter is supposed to be part of ARFLAGS.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5386)