x86/tss: Fix clang build following c/s 7888440625

Clang-3.5 from Debian Jessie fails with:

  smpboot.c:829:29: error: statement expression not allowed at file scope
          BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
                              ^
  /local/xen.git/xen/include/asm/percpu.h:14:7: note: expanded from macro
          'this_cpu'
      (*RELOC_HIDE(&per_cpu__##var, get_cpu_info()->per_cpu_offset))
        ^
  /local/xen.git/xen/include/xen/compiler.h:98:3: note: expanded from macro
          'RELOC_HIDE'
    ({ unsigned long __ptr;                       \
    ^
  /local/xen.git/xen/include/xen/lib.h:26:53: note: expanded from macro
          'BUILD_BUG_ON'
  #define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
                                                      ^
  /local/xen.git/xen/include/xen/lib.h:25:57: note: expanded from macro
          'BUILD_BUG_ON_ZERO'
  #define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
                                                          ^
  1 error generated.
  /local/xen.git/xen/Rules.mk:202: recipe for target 'smpboot.o' failed

This is obviously a compiler bug because the BUILD_BUG_ON() is not at file
scope.  However, it can be worked around by using a local variable.

Spotted by Gitlab CI.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wl@xen.org>
master commit: 1722da6c0c
master date: 2019-08-14 12:04:20 +0100

xen/arch/x86/smpboot.c

@@ -833,9 +833,11 @@ static int setup_cpu_root_pgt(unsigned int cpu)
         rc = clone_mapping(idt_tables[cpu], rpt);
     if ( !rc )
     {
-        BUILD_BUG_ON(sizeof(this_cpu(tss_page)) != PAGE_SIZE);
+        struct tss_page *ptr = &per_cpu(tss_page, cpu);
 
-        rc = clone_mapping(&per_cpu(tss_page, cpu).tss, rpt);
+        BUILD_BUG_ON(sizeof(*ptr) != PAGE_SIZE);
+
+        rc = clone_mapping(&ptr->tss, rpt);
     }
     if ( !rc )
         rc = clone_mapping((void *)per_cpu(stubs.addr, cpu), rpt);