x86: fix tsc unification buglet with ftrace and stackprotector

Yinghai Lu reported crashes on 64-bit x86:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
 IP: [<ffffffff80253b17>] hrtick_start_fair+0x89/0x173
 [...]

And with a long session of debugging and a lot of difficulty, tracked it down
to this commit:

 --------------->
 8fbbc4b4 is first bad commit
 commit 8fbbc4b4
 Author: Alok Kataria <akataria@vmware.com>
 Date:   Tue Jul 1 11:43:34 2008 -0700

     x86: merge tsc_init and clocksource code
 <--------------

The problem is that the TSC unification missed these Makefile rules
in arch/x86/kernel/Makefile:

  # Do not profile debug and lowlevel utilities
  CFLAGS_REMOVE_tsc_64.o = -pg
  CFLAGS_REMOVE_tsc_32.o = -pg
  ...
  CFLAGS_tsc_64.o         := $(nostackp)
  ...

which rules make sure that various instrumentation and debugging
facilities are disabled for code that might end up in a VDSO - such as
the TSC code.
Reported-and-bisected-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

Conflicts:
Signed-off-by: Ingo Molnar <mingo@elte.hu>

arch/x86/kernel/Makefile

@@ -13,7 +13,7 @@ CPPFLAGS_vmlinux.lds += -U$(UTS_MACHINE)
 nostackp := $(call cc-option, -fno-stack-protector)
 CFLAGS_vsyscall_64.o	:= $(PROFILING) -g0 $(nostackp)
 CFLAGS_hpet.o		:= $(nostackp)
-CFLAGS_tsc_64.o		:= $(nostackp)
+CFLAGS_tsc.o		:= $(nostackp)
 
 obj-y			:= process_$(BITS).o signal_$(BITS).o entry_$(BITS).o
 obj-y			+= traps_$(BITS).o irq_$(BITS).o