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
new file mode 100644
index 0000000..cc53ab4
--- /dev/null
+++ b/libc/arch-mips/syscalls/__sigaction.S
@@ -0,0 +1,23 @@
+/* Generated by gensyscalls.py. Do not edit. */
+
+#include <asm/unistd.h>
+    .text
+    .globl __sigaction
+    .align 4
+    .ent __sigaction
+
+__sigaction:
+    .set noreorder
+    .cpload $t9
+    li $v0, __NR_sigaction
+    syscall
+    bnez $a3, 1f
+    move $a0, $v0
+    j $ra
+    nop
+1:
+    la $t9,__set_errno
+    j $t9
+    nop
+    .set reorder
+    .end __sigaction