Silence -Wmaybe-uninitialized warning from target_waitstatus

Currently, one use of target_waitstatus yields a warning:

     target/waitstatus.h: In function 'void stop_all_threads()':
     target/waitstatus.h:175:13: warning: 'ws.target_waitstatus::m_value' may be used uninitialized in this function [-Wmaybe-uninitialized]
       175 |     m_value = other.m_value;
	   |     ~~~~~~~~^~~~~~~~~~~~~~~

This patch silences the warning.  I tried the "volatile member"
approach that was used for gdb::optional, but that didn't work, so
this patch simply initializes the member.