[libprefix] Add addr32 prefix required by older assemblers

Explicitly state that we are using 32-bit addressing in 16-bit code.

GNU as 2.15 (FreeBSD/amd64 7-STABLE) got confused that 32-bit registers
are used in the code that was declared as 16-bit.  Add explicit modifier
'addr32' to make assembler happy.

Signed-off-by: Eygene Ryabinkin <rea-fbsd@codelabs.ru>

src/arch/i386/prefix/libprefix.S

 	movw	%ss, %ax
 	shll	$4, %eax
 	movzwl	%bp, %edi
-	leal	PM_CALL_VAR(gdt)(%eax, %edi), %eax
+	addr32 leal PM_CALL_VAR(gdt)(%eax, %edi), %eax
 	movl	%eax, PM_CALL_VAR(gdt_base)(%bp)
 	movw	%cs, %ax
 	movw	$PM_CALL_VAR(pm_cs), %di