gdb: Fix GDB_AC_CHECK_BFD macro regression

gdb: Fix GDB_AC_CHECK_BFD macro regression

Commit 5218fa9e8937b007d554f1e01c2e4ecdb9b7e271, "gdb: use libtool in
GDB_AC_CHECK_BFD" dropped passing in existing LDFLAGS. In our environment,
this caused the configure check "checking for ELF support in BFD" to stop
working causing build failures as we need our LDFLAGS to be used for
correct linking.

That change also meant the code failed to match the comments. Add back the
missing LDFLAGS preservation, fix our builds and match the comment.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Change-Id: Ie91509116fab29f95b9db1ff0b6ddc280d460112
Approved-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-By: Jose E. Marchesi <jose.marchesi@oracle.com>