stats: Create HDF5 stat files relative to simout

stats: Create HDF5 stat files relative to simout

When using the HDF5 stats backend, we currently pass the filename from
the command line straight to the HDF5 library. This behaviour is
different from the expected behaviour when using plain text stat files
where relative paths are resolved relative to simout. This change adds
support for resolving hdf5 paths relative to simout.

Change-Id: I09cb0b7468735e697125eda7f04c5a85725ca8d0
Signed-off-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20508
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>