stats: Don't output index in vectors of length 1

stats: Don't output index in vectors of length 1

New-world stats didn't implement the old gem5 quirk where the index of
an element in a vector of size 1 is ignored. This affects the object
path when printed in a stat file. Implement the same quirk as the
original code to make sure names of old- and new-world stats are
compatible.

Change-Id: I12696d40baaed2d0bb1ef061e87c3b1d0f4bb4b1
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/21163
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>