Kernel loading address seems like important parameter that somehow got scrambled up while developing amd64.
This change line-up amd64 with the i386 and the rest of supported architectures by defining KERNLOAD in the vmparam.h and getting rid of magic constant in the linker script.
Use that constant now to eliminate another magic constant 200 lines deep inside unassuming TU "copy.c" 3 levels deep in the loader tree.