mem-cache: Fix integer promotion of mask

mem-cache: Fix integer promotion of mask

When applying the bitwise not to a short integer the compiler
automatically promotes it to an integer. For example, if a 8-bit
mask=0xFF, and the compiler decides to promote the mask to 32-bit
to apply the bitwise not, ~mask=0xFFFFFF00, which will yield wrong
results for popcount(): expected=0, got=24.

Change-Id: I95efba5532c27ca004ff6947d4b51a8a14f09741
Signed-off-by: Daniel R. Carvalho <odanrc@yahoo.com.br>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/33374
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>