x86/asm/tsc: Add rdtscll() merge helper

Some in-flight code makes use of the old rdtscll() (now removed), provide a wrapper
for a kernel cycle to smooth the transition to rdtsc().

( We use the safest variant, rdtsc_ordered(), which has barriers - this adds another
  incentive to remove the wrapper in the future. )

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Huang Rui <ray.huang@amd.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm ML <kvm@vger.kernel.org>
Link: http://lkml.kernel.org/r/dddbf98a2af53312e9aa73a5a2b1622fe5d6f52b.1434501121.git.luto@kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

arch/x86/include/asm/msr.h

@@ -152,6 +152,9 @@ static __always_inline unsigned long long rdtsc_ordered(void)
 	return rdtsc();
 }
 
+/* Deprecated, keep it for a cycle for easier merging: */
+#define rdtscll(now)	do { (now) = rdtsc_ordered(); } while (0)
+
 static inline unsigned long long native_read_pmc(int counter)
 {
 	DECLARE_ARGS(val, low, high);