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
similarity index 89%
rename from libc/arch-arm/syscalls/sigaction.S
rename to libc/arch-arm/syscalls/__sigaction.S
index 65e8840..23a2b23 100644
--- a/libc/arch-arm/syscalls/sigaction.S
+++ b/libc/arch-arm/syscalls/__sigaction.S
@@ -4,7 +4,7 @@
 #include <linux/err.h>
 #include <machine/asm.h>
 
-ENTRY(sigaction)
+ENTRY(__sigaction)
     mov     ip, r7
     ldr     r7, =__NR_sigaction
     swi     #0
@@ -13,4 +13,4 @@
     bxls    lr
     neg     r0, r0
     b       __set_errno
-END(sigaction)
+END(__sigaction)