Mark pieces of values as unavailable if the corresponding memory

	Mark pieces of values as unavailable if the corresponding memory
	is unavailable.

	gdb/
	* valops.c: Include tracepoint.h.
	(value_fetch_lazy): Use read_value_memory.
	(read_value_memory): New.
	* value.h (read_value_memory): Declare.
	* dwarf2loc.c (read_pieced_value): Use read_value_memory.
	* exec.c (section_table_available_memory): New function.
	* exec.h (section_table_available_memory): Declare.