Unobfuscate "KERNLOAD" parameter on amd64. This change lines-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, which albeit
documented via comment but isn't programmatically accessible at a compile time.

Use KERNLOAD to eliminate another (matching) magic constant 100 lines down
inside unremarkable TU "copy.c" 3 levels deep in the EFI loader tree.

Reviewed by: markj
Approved by: markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D27355