base: Add an asymmetrical Coroutine class

base: Add an asymmetrical Coroutine class

This patch is providing gem5 a Coroutine class to be used for
instantiating asymmetrical coroutines. Coroutines are built on top of
gem5 fibers, which makes them ucontext based.

Change-Id: I7bb673a954d4a456997afd45b696933534f3e239
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/11195
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>