Fix regression on Windows with WOW64

Fix regression on Windows with WOW64

Internally at AdaCore, we recently started testing a 64-bit gdb
debugging 32-bit processes.  This failed with gdb head, but not with
gdb 11.

The tests fail like this:

     Starting program: [...].exe
     warning: Could not load shared library symbols for WOW64_IMAGE_SECTION.
     Do you need "set solib-search-path" or "set sysroot"?
     warning: Could not load shared library symbols for WOW64_IMAGE_SECTION.
     Do you need "set solib-search-path" or "set sysroot"?
     warning: Could not load shared library symbols for NOT_AN_IMAGE.
     Do you need "set solib-search-path" or "set sysroot"?
     warning: Could not load shared library symbols for NOT_AN_IMAGE.
     Do you need "set solib-search-path" or "set sysroot"?

After some debugging and bisecting, to my surprise the bug was
introduced by commit 183be222 ("gdb, gdbserver: make target_waitstatus
safe").

The problem occurs in handle_exception.  Previously the code did:

    -  ourstatus->kind = TARGET_WAITKIND_STOPPED;
    [...]
	 case EXCEPTION_BREAKPOINT:
    [...]
    -	  ourstatus->kind = TARGET_WAITKIND_SPURIOUS;
    [...]
	   /* FALLTHROUGH */
	 case STATUS_WX86_BREAKPOINT:
	   DEBUG_EXCEPTION_SIMPLE ("EXCEPTION_BREAKPOINT");
    -      ourstatus->value.sig = GDB_SIGNAL_TRAP;
    [...]
    -  last_sig = ourstatus->value.sig;

However, in the new code, the fallthrough case does:

    +      ourstatus->set_stopped (GDB_SIGNAL_TRAP);

... which changes the 'kind' in 'ourstatus' after falling through.

This patch rearranges the 'last_sig' setting to more closely match
what was done before (this is probably not strictly needed but also
seemed harmless), and removes the fall-through in the
'ignore_first_breakpoint' case when __x86_64__ is defined.