x86: Put back -pg to tsc.o and add no GCOV to vread_tsc_64.o

The commit 44259b1a
    Author: Andy Lutomirski <luto@MIT.EDU>
    x86-64: Move vread_tsc into a new file with sensible options

Removed the -pg from tsc.o which caused the function graph tracer
to go into an infinite function call recursion as it uses the tsc
internally outside its recursion protection, thus tracing the tsc
breaks the function graph tracer.

This commit also added the file vread_tsc_64.c that gets used
by vdso but failed to prevent GCOV from monkeying with it,
causing userspace to try to access kernel data when GCOV was
enabled.

Thanks to Thomas Gleixner for pointing out GCOV as the likely
culprit that added strange kernel accesses into the vread_tsc()
call.

Cc: Author: Andy Lutomirski <luto@MIT.EDU>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>