set sysroot command on AIX has no effect if a program depends on shared
library archives (.a). Fixed by using solib_find and solib_bfd_fopen
instead of gdb_bfd_open in solib_aix_bfd_open.
* solib-aix.c (solib_aix_bfd_open): Call solib_find so that sysroot
path is set properly if program has a dependency on .a archive and
sysroot is set via set sysroot command.