amd64: move efi_map_regs definition

Since efi_map_regs is used in machdep.c, efirt being compiled as a
module causes it to be left undefined.

Fixes: 85dcdb7aad85498b3f497b8752e69b8612b27cb7
Noticed by: kib
Reviewed by: kib, markj
Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D51183