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/arch-arm/syscalls.mk b/libc/arch-arm/syscalls.mk
index 315d730..f9d348b 100644
--- a/libc/arch-arm/syscalls.mk
+++ b/libc/arch-arm/syscalls.mk
@@ -22,6 +22,7 @@
 syscall_src += arch-arm/syscalls/__rt_sigtimedwait.S
 syscall_src += arch-arm/syscalls/__sched_getaffinity.S
 syscall_src += arch-arm/syscalls/__set_tls.S
+syscall_src += arch-arm/syscalls/__sigaction.S
 syscall_src += arch-arm/syscalls/__statfs64.S
 syscall_src += arch-arm/syscalls/__sys_clone.S
 syscall_src += arch-arm/syscalls/__syslog.S
@@ -178,7 +179,6 @@
 syscall_src += arch-arm/syscalls/setuid.S
 syscall_src += arch-arm/syscalls/setxattr.S
 syscall_src += arch-arm/syscalls/shutdown.S
-syscall_src += arch-arm/syscalls/sigaction.S
 syscall_src += arch-arm/syscalls/sigaltstack.S
 syscall_src += arch-arm/syscalls/signalfd4.S
 syscall_src += arch-arm/syscalls/socket.S