crt: remove noreturn attribute for _assert and _wassert

Since msvcrt40.dll, applications can call _set_error_mode(_OUT_TO_MSGBOX)
to make assert() and abort() display messages in a message box instead of
printing them to stderr; when doing so, user can press "Ignore" button to
continue after an assertion has failed.

Both _assert and _wassert are declared with noreturn attribute, which would
result in undefined behavior in case when they return to the caller site.

Signed-off-by: LIU Hao <lh_mouse@126.com>