sparc: remove unneeded atomic implementation

The definition in SupportDefs.h using gcc builtins is sufficient. No
need for a custom one. The same approach is used on x86 with gcc8
already, but other platforms had not been adjusted to use it.

Change-Id: I3973ff723a31f90cc8d19ac098eb1e85d471d610
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3594
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

sparc: add defines and minimum set of required files

Gets the stage0 bootstrap to run.
Imlementation is probably nonsense at this point.

Change-Id: I10876efbb54314b864c0ad951152757cdb2fd366
Reviewed-on: https://review.haiku-os.org/c/1061
Reviewed-by: waddlesplash <waddlesplash@gmail.com>