The bootloader self relocation code was slightly wrong for the
R_AARCH64_RELATIVE relocation found on arm64. It would try to add the
contents of the memory location being relocated to the base address and
the relocation addend. This worked when the contents was zero, however
this now seems to be set to the value of the addend so we add this twice.
Fix this by just setting the memory to the computed value.

Reviewed by: emaste
MFC after: 1 week
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8177