Add low_new_clone method to linux_nat_target.

This patch adds a low_new_clone method to linux_nat_target, called after
a PTRACE_EVENT_CLONE is detected, similar to how low_new_fork is called
after PTRACE_EVENT_(V)FORK.

This is useful for targets that need to copy state associated with a
thread that is inherited across clones.

gdb/ChangeLog:
2020-03-30  Pedro Franco de Carvalho  <pedromfc@linux.ibm.com>

	* linux-nat.h (low_new_clone): New method.
	* linux-nat.c (linux_handle_extended_wait): Call low_new_clone.