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)

Configurations/README

diff --git a/Configurations/README b/Configurations/README
index 26cd0c6..8d32a25 100644
--- a/Configurations/README
+++ b/Configurations/README
@@ -95,7 +95,8 @@
                            (NOTE: this is here for future use, it's
                            not implemented yet)
         arflags         => Flags to be used with the library archive
-                           command.
+                           command.  On Unix, this includes the
+                           command letter, 'r' by default.
 
         ranlib          => The library archive indexing command, the
                            default is 'ranlib' it it exists.