AIX: Do not use -brtl to create shared libraries (#13997)

The flag was added incorrectly by commit 9c3a6eb4 (Need -brtl when creating
shared libraries, 2003-05-16).  According to "man ld" the -G option implies
"-brtl -bnortllib ...", -brtl implies "-brtllib", and -brtllib should only be
used for executables, not shared libraries.  Therefore it is incorrect and
unnecessary to specify -brtl explicitly after -G.

Reported-by: Kevin Burge <kcburge@gmail.com>
Merge topic 'aix-shared-flags' into next

bce7a2a AIX: Do not use -brtl to create shared libraries (#13997)