loader: restructure loader source tree
It has been eating me some time now - the loader tree is a bit of mess and should be cleaned up a bit (and synced with FreeBSD version a bit).
1. move libstand to usr/src/boot/libsa and merge with libc bits. Remove unsupported architectures while there.
2. move usr/src/boot/sys/boot/* to usr/src/boot, drop 2 levels of tree.
3. move smbios.[ch] from libi386 to libsa. smbios detection code is not specific to BIOS systems and other systems need it too. (allows loader.efi to stop depend on libi386).
4. forth scripts and config files are installed from usr/src/boot/forth (moved from i386/loader). Those files are not unique to BIOS loader.
While there is a bit code shuffling related to smbios, there are no functional changes.
Testing done: build/install/boot. smbios data is exported to loader environment, boot is working as expected. Both BIOSand UEFI boot is tested.