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/__sigaction.S b/libc/arch-arm/syscalls/__sigaction.S
new file mode 100644
index 0000000..23a2b23
--- /dev/null
+++ b/libc/arch-arm/syscalls/__sigaction.S
@@ -0,0 +1,16 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <asm/unistd.h>
+#include <linux/err.h>
+#include <machine/asm.h>
+
+ENTRY(__sigaction)
+    mov     ip, r7
+    ldr     r7, =__NR_sigaction
+    swi     #0
+    mov     r7, ip
+    cmn     r0, #(MAX_ERRNO + 1)
+    bxls    lr
+    neg     r0, r0
+    b       __set_errno
+END(__sigaction)