tcg: Move tci_tb_ptr to -common

This requires global visibility to common code. Move to tcg-common.

Cc: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-Id: <cb0340eba225ab4945aa6cf7c9013f33aa05bcf8.1441614289.git.crosthwaite.peter@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>