stand/amd64: remove unused addr parameter from bi_load64

All callers of bi_load64 pass 0 as the addr parameter, so just remove
it and always calculate the last load address from the module chain.

No functional change.

Sponsored by: Citrix Systems R&D
Reviewed by: tsoome, imp
Differential revision: https://reviews.freebsd.org/D28412

(cherry picked from commit 45ecda8ebf80bb341aa3b4955810518ae6fab158)