x86/entry: fix jump into restore_all_guest without %rbx correctly set

e047b8d0fa went too far when limiting obtaining the vCPU pointer.  While the
code in ist_dispatch_done does indeed only need the vCPU pointer when PV32 is
enabled, the !PV32 path will end up jumping into restore_all_guest which does
require rbx == vCPU pointer.

Fix by moving the fetching of the vCPU pointer to be done outside of the PV32
code block.

Fixes: e047b8d0fa ('x86/entry: replace two GET_CURRENT() uses')
Reported-by: Edwin Torok <edwin.torok@cloud.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/x86_64/entry.S

@@ -1139,8 +1139,9 @@ FUNC(handle_ist_exception)
         movl  $EVENT_CHECK_VECTOR,%edi
         call  send_IPI_self
 1:
-#ifdef CONFIG_PV32
+        /* For restore_all_guest. */
         mov   STACK_CPUINFO_FIELD(current_vcpu)(%r14), %rbx
+#ifdef CONFIG_PV32
         movq  VCPU_domain(%rbx),%rax
         cmpb  $0,DOMAIN_is_32bit_pv(%rax)
         je    restore_all_guest