Fix quoting in gdb-add-index.sh

When the filename quoting change was merged into the AdaCore tree, we
saw a regression in a test setup that uses the DWARF 5 index (that is
running gdb-add-index), and a filename with a space in it.

Initially I thought this was a change in the 'file' command -- but
looking again, I found out that 'file' has worked this way for a
while, and our immediate error was caused by the (documented) change
to "save gdb-index".

While I'm not sure why this test was working previously, it seems to
me that gdb-add-index.sh requires a change to quote the arguments to
"file" and "save gdb-index".

While working on this, though, it seemed to me that multiple other
spots needed quoting for the script to work correctly.  And, I was
unable to get quoting working correctly in the objcopy calls, so I
split it into multiple different invocations.

Approved-by: Kevin Buettner <kevinb@redhat.com>