runtime_loader: define _LOADER_MODE for arch directories

Change-Id: I7eb6b77502eed76a529657d9ec733b72c9fc89c0
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3275
Reviewed-by: Jérôme Duval <jerome.duval@gmail.com>

sparc: link the runtime_loader

Change-Id: Id30476480f4e4141935d6735209ecc0872dd6696
Reviewed-on: https://review.haiku-os.org/c/1367
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

sparc: fix bootloader build

- Add various missing jamfiles
- Add required implementation stubs
- Update openfirmware jamfiles for multiboot support
- Update linker rules for sparc loader

Change-Id: I2d06c7a4d33827f58d82946687003f9a0dcb1b7d
Reviewed-on: https://review.haiku-os.org/c/1329
Reviewed-by: waddlesplash <waddlesplash@gmail.com>