Fix mip64 build.

Fallout from the unified sysroot work.

Bug: N/A
Test: builds
Change-Id: If0595a241b9ce0d8c8c7137ddaf8fca932487b7c
diff --git a/libc/arch-mips64/syscalls/sigaltstack.S b/libc/arch-mips64/syscalls/sigaltstack.S
index 92778c1..13b539b 100644
--- a/libc/arch-mips64/syscalls/sigaltstack.S
+++ b/libc/arch-mips64/syscalls/sigaltstack.S
@@ -5,21 +5,21 @@
 ENTRY(sigaltstack)
     .set push
     .set noreorder
-    li v0, __NR_sigaltstack
+    li $v0, __NR_sigaltstack
     syscall
-    bnez a3, 1f
-    move a0, v0
-    j ra
+    bnez $a3, 1f
+    move $a0, $v0
+    j $ra
     nop
 1:
-    move t0, ra
-    bal     2f
+    move $t0, $ra
+    bal 2f
     nop
 2:
-    .cpsetup ra, t1, 2b
-    LA t9,__set_errno_internal
+    .cpsetup $ra, $t1, 2b
+    LA $t9, __set_errno_internal
     .cpreturn
-    j t9
-    move ra, t0
+    j $t9
+    move $ra, $t0
     .set pop
 END(sigaltstack)