base: Fix fiber compilation issue for Mac OS

base: Fix fiber compilation issue for Mac OS

Mac OS has deprecated the use of ucontext and at the moment we are
using a workaround to enable it. A side-effect is that the code is
marked as _POSIX_C_SOURCE and consequently <sys/mman.h> requires the
code to be also marked as _DARWIN_C_SOURCE to include the definition
of MAP_ANONYMOUS.

Change-Id: I65550d11a0a21cd36d832a7de6320e7e3f332a9d
Signed-off-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/14817
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>