mboot64: Boot 64-bit ELF executables.

RTSM64 # semiload 82000000 hello-aarch64.axf
Loading file "hello-aarch64.axf"
67324 bytes read
RTSM64 # bootelf -p 82000000

HELLO
RTSM64 #

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>