Define generic memory type.

Define generic memory type.

Generic system memory is cached on all known ARMv8 and x86 CPUs, so
advertise it as such.

Bug b/118383648

Change-Id: Ic757206a497fc299e1e76a939220092f7c0a124d
Reviewed-on: https://swiftshader-review.googlesource.com/c/22448
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Corentin Wallez <cwallez@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>