Fix sigaction(3) for 64-bit.

Also clean up <signal.h> and revert the hacks that were necessary
for 64-bit in linker/debugger.cpp until now.

Change-Id: I3b0554ca8a49ee1c97cda086ce2c1954ebc11892
diff --git a/libc/SYSCALLS.TXT b/libc/SYSCALLS.TXT
index f5712e1..e5e3c4d 100644
--- a/libc/SYSCALLS.TXT
+++ b/libc/SYSCALLS.TXT
@@ -224,7 +224,7 @@
 int           timerfd_gettime(int, struct itimerspec*)   all
 
 # signals
-int     sigaction(int, const struct sigaction*, struct sigaction*)  arm,x86,mips
+int     __sigaction:sigaction(int, const struct sigaction*, struct sigaction*)  arm,mips,x86
 int     __rt_sigaction:rt_sigaction(int, const struct sigaction*, struct sigaction*, size_t)  all
 int     __rt_sigpending:rt_sigpending(sigset_t*, size_t)  all
 int     __rt_sigprocmask:rt_sigprocmask(int, const sigset_t*, sigset_t*, size_t)  all