cr: generalize the type to store the value of the TLS register

Supported machine architectures provide TLS stogares of different sizes:
the size of the TLS storage in x86-64 is 24 bytes, ARM --- 4 bytes
and upcoming AArch64 --- 8 bytes. This means every supported architecture
needs a specific type to store the value of the TLS register.

This patch reworks the insterface of the routines arch_get_tls()
and restore_tls() passing them the TLS storage by pointer
rather than by value to simplify the TLS stub for x86.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Reviewed-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>