debuggerd: rethrow the full signal we receive, always.

The previous code assumed that returning would be sufficient to rethrow
signals like SIGSEGV. This is not true, for example,  in the case where a
SIGSEGV is sent via kill(2). We were previously only sending the signal
to ourselves in some cases, because using kill(2) would lose information
in the siginfo_t argument. Use rt_tgsigqueueinfo(2) instead to preserve
its contents.

Bug: http://b/27367422
Change-Id: I1be822818d5905461979c7e12dc4e9c25049273b