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-mips/syscalls/sigaction.S b/libc/arch-mips/syscalls/__sigaction.S
similarity index 78%
rename from libc/arch-mips/syscalls/sigaction.S
rename to libc/arch-mips/syscalls/__sigaction.S
index d1808c6..cc53ab4 100644
--- a/libc/arch-mips/syscalls/sigaction.S
+++ b/libc/arch-mips/syscalls/__sigaction.S
@@ -2,11 +2,11 @@
 
 #include <asm/unistd.h>
     .text
-    .globl sigaction
+    .globl __sigaction
     .align 4
-    .ent sigaction
+    .ent __sigaction
 
-sigaction:
+__sigaction:
     .set noreorder
     .cpload $t9
     li $v0, __NR_sigaction
@@ -20,4 +20,4 @@
     j $t9
     nop
     .set reorder
-    .end sigaction
+    .end __sigaction