Avoid scopes.exp failure on certain architectures

A buildbot pointed out that my changes to gdb.dap/scopes.exp caused a
test failure.  The new code iterates over all the registers, fetching
their children (when possible).  The failure comes because this code
failed to consider that a register might have "indexed" children,
which I believe can occur when a register is vector-like.

This patch fixes the problem by arranging to fetch indexed children
when indicated.