Use std::unique_ptr in reg_buffer

Use std::unique_ptr in reg_buffer

Using std::unique_ptr allows to remove the manual xfree in the
destructor.

If I understand correctly, using the () after the new operator will make
sure the allocated objects will be value initialized, which for scalars
means they are zero-initialized.  So it should have the same behavior as
XCNEWVEC.

gdb/ChangeLog:

	* regcache.h (reg_buffer) <~reg_buffer>: Use default destructor.
	<m_registers, m_register_status>: Change type to
	std::unique_ptr.
	* regcache.c (reg_buffer::reg_buffer): Use new instead of
	XCNEWVEC.