x86/compat: fix compilation errors with clang 6

x86/compat: fix compilation errors with clang 6

The following errors are generated when compiling Xen with clang 6:

In file included from x86_64/asm-offsets.c:9:
In file included from /root/src/xen/xen/include/xen/sched.h:8:
In file included from /root/src/xen/xen/include/xen/shared.h:6:
In file included from /root/src/xen/xen/include/compat/arch-x86/../xen.h:9:
/root/src/xen/xen/include/compat/arch-x86/xen.h:10:10: error: the current #pragma pack aligment
      value is modified in the included file [-Werror,-Wpragma-pack]
#include "xen-x86_32.h"
         ^
/root/src/xen/xen/include/compat/arch-x86/xen-x86_32.h:40:9: note: previous '#pragma pack'
      directive that modifies alignment is here
#pragma pack()
        ^
In file included from x86_64/asm-offsets.c:9:
In file included from /root/src/xen/xen/include/xen/sched.h:8:
In file included from /root/src/xen/xen/include/xen/shared.h:6:
/root/src/xen/xen/include/compat/arch-x86/../xen.h:9:10: error: the current #pragma pack aligment
      value is modified in the included file [-Werror,-Wpragma-pack]
#include "arch-x86/xen.h"
         ^
/root/src/xen/xen/include/compat/arch-x86/xen.h:71:9: note: previous '#pragma pack' directive that
      modifies alignment is here
#pragma pack()
        ^
2 errors generated.

Fix this by using pragma push/pop in order to store the current pragma
value in the compiler stack and later restoring it when using clang.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>