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>

src/base/SConscript

@@ -48,6 +48,7 @@ if env['USE_PNG']:
     Source('pngwriter.cc')
 Source('fiber.cc')
 GTest('fibertest', 'fibertest.cc', 'fiber.cc')
+GTest('coroutinetest', 'coroutinetest.cc', 'fiber.cc')
 Source('framebuffer.cc')
 Source('hostinfo.cc')
 Source('inet.cc')