trampoline for linux on 64-bit platform
* conf/x86_64-efi.rmk (linux_mod_SOURCES): added
loader/i386/efi/linux_trampoline.S
* include/grub/x86_64/efi/loader.h (grub_linux_real_boot): removed
declration
* kern/x86_64/efi/startup.S (grub_linux_real_boot): moved from here
* loader/i386/linux_trampoline.S: moved here
* loader/i386/efi/linux.c (allocate_pages): reserve space for trampoline
(jumpvector): removed
(grub_linux_trampoline_start): new declaration
(grub_linux_trampoline_end): likewise
(grub_linux_boot): use trampoline when on 64-bit platform
* loader/i386/linux.c: likewise
