x86/CET: Fix build following c/s 43b98e7190

x86/CET: Fix build following c/s 43b98e7190

OSSTest reports:

  x86_64.S: Assembler messages:
  x86_64.S:57: Error: no such instruction: `setssbsy'
  /home/osstest/build.150510.build-amd64/xen/xen/Rules.mk:183: recipe for target 'head.o' failed
  make[4]: Leaving directory '/home/osstest/build.150510.build-amd64/xen/xen/arch/x86/boot'
  make[4]: *** [head.o] Error 1

All use of CET instructions, even those inside alternative blocks, needs to be
behind CONFIG_XEN_SHSTK, as it indicates suitable toolchain support.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>