Bring back position dependent ubldr.

In D5512#119297, @mst_semihalf.com wrote:

Would it be possible to leave the position-dependent targets in the makefile as it was on arm32? The '-pie' version is rejected by U-Boot when bootelf is run, as it only recognizes ELFs with e_type == ET_EXEC. So in this version only the .bin is usable with U-Boot.

Add comment in ldscript.aarch64 explaining text section alignment.

Get relocation to work. Remove static linked target.

Don't adjust va->pa offset in EFI loader. Clean up assembly in start.S.