lib: backtrace printing

Functions to walk stack and print backtrace. The stack's unadorned as

	STACK: [@]addr addr addr ...

where the optional @ indicates that addr isn't a return address.

A follow-up patch post-processes the output to pretty-print the stack.

Frame stack walker is just a stub on arm and ppc.

Signed-off-by: Peter Feiner <pfeiner@google.com>
Reviewed-By: Andrew Jones <drjones@redhat.com>
Message-Id: <adc65c3511125ea46f0fe53a2fc53a062ada7924.1458689655.git.pfeiner@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>