Fix set sysroot command on AIX

Fix set sysroot command on AIX

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.

gdb/
2016-10-14  Sangamesh Mallayya  <sangamesh.swamy@in.ibm.com>
	    Ulrich Weigand  <uweigand@de.ibm.com>

	* 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.

Signed-off-by: Ulrich Weigand <ulrich.weigand@de.ibm.com>