xen: make -fstack-protector work under Xen

-fstack-protector uses a special per-cpu "stack canary" value.
gcc generates special code in each function to test the canary to make
sure that the function's stack hasn't been overrun.

On x86-64, this is simply an offset of %gs, which is the usual per-cpu
base segment register, so setting it up simply requires loading %gs's
base as normal.

On i386, the stack protector segment is %gs (rather than the usual kernel
percpu %fs segment register).  This requires setting up the full kernel
GDT and then loading %gs accordingly.  We also need to make sure %gs is
initialized when bringing up secondary cpus too.

To keep things consistent, we do the full GDT/segment register setup on
both architectures.

Because we need to avoid -fstack-protected code before setting up the GDT
and because there's no way to disable it on a per-function basis, several
files need to have stack-protector inhibited.

[ Impact: allow Xen booting with stack-protector enabled ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/mm/Makefile

@@ -1,6 +1,10 @@
 obj-y	:=  init.o init_$(BITS).o fault.o ioremap.o extable.o pageattr.o mmap.o \
 	    pat.o pgtable.o gup.o
 
+# Make sure __phys_addr has no stackprotector
+nostackp := $(call cc-option, -fno-stack-protector)
+CFLAGS_ioremap.o		:= $(nostackp)
+
 obj-$(CONFIG_SMP)		+= tlb.o
 
 obj-$(CONFIG_X86_32)		+= pgtable_32.o iomap_32.o