sparc: fill in kernel entry code

- Fix elf32 vs elf64 (copied from EFI)
- Write the kernel start code

Change-Id: I6133185681307ecabc617b47b36551e52f9ac2c9
Reviewed-on: https://review.haiku-os.org/c/haiku/+/3578
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>