cpu: Never use a empty byteEnable

cpu: Never use a empty byteEnable

The byteEnable variable is used for masking bytes in a memory request.
The default behaviour is to provide from the ExecContext to the CPU
(and then to the LSQ) an empty vector, which is the same as providing
a vector where every element is true.
Such vectors basically mean: do not mask any byte in the memory request.

This behaviour adds more complexity to the downstream LSQs, which now
have to distinguish between an empty and non-empty byteEnable.

This patch is simplifying things by transforming an empty vector into
a all true one, making sure the CPUs are always receiving a non empty
byteEnable.

JIRA: https://gem5.atlassian.net/browse/GEM5-196

Change-Id: I1d1cecd86ed64c53a314ed700f28810d76c195c3
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23285
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>