x86/boot: call reloc() using stdcall calling convention

x86/boot: call reloc() using stdcall calling convention

Current reloc() call method makes confusion and does not scale well
for more arguments. And subsequent patch adding multiboot2 protocol
support have to pass 3 arguments instead of 2. Hence, move reloc()
call to stdcall calling convention. One may argue that we should use
standard cdecl calling convention. However, stdcall is better here
than cdecl because we do not need to remove "manually" arguments from
stack in xen/arch/x86/boot/head.S assembly file.

Suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>