_signal was static in 64 bit, and hidden on 32 bit. There is no
reason to have this distinction, so make it hidden in all cases.
Change-Id: I09d5d93ac8cab4fe14dc7bdfeb25aa46a3b7413d
Also clean up <signal.h> and revert the hacks that were necessary
for 64-bit in linker/debugger.cpp until now.
Change-Id: I3b0554ca8a49ee1c97cda086ce2c1954ebc11892