x86: use EFI calling convention for efi_main on x86_64

UEFI specifies the calling convention used in Microsoft compilers;
first arguments of a function are passed in (%rcx, %rdx, %r8, %r9).

All other compilers use System V ABI by default, passing first integer
arguments of a function in (%rdi, %rsi, %rdx, %rcx, %r8, %r9).

These ABI also specify different sets of registers that must be preserved
across function calls (callee-saved).

GCC allows using the Microsoft calling convention by adding the ms_abi
attribute to a function declaration.

Current EFI implementation in U-Boot specifies EFIAPI for efi_main()
in the test apps but uses default calling convention in lib/efi.

Save efi_main() arguments in the startup code on x86_64;
use EFI calling convention for _relocate() on x86_64;
consistently use EFI calling convention for efi_main() everywhere.

Signed-off-by: Ivan Gorinov <ivan.gorinov@intel.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>