x86/guest: Fix assembler warnings with newer binutils

x86/guest: Fix assembler warnings with newer binutils

GAS of at least version 2.34 complains:

  hypercall_page.S: Assembler messages:
  hypercall_page.S:24: Warning: symbol 'HYPERCALL_set_trap_table' already has its type set
  ...
  hypercall_page.S:71: Warning: symbol 'HYPERCALL_arch_7' already has its type set

which is because the whole page is declared as STT_OBJECT already.  Rearrange
.set with respect to .type in DECLARE_HYPERCALL() so STT_FUNC is already in
place.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>