Export some symbols for vendor libmemunreachable

Add the malloc symbols used by libmemunreachable to the VNDK, and
make libc_malloc_debug_backtrace vendor_available.

Bug: 132302484
Test: m checkbuild
Change-Id: Ide555195afa084c13eaeaf0eab6ff90787b5f2d0
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index bc26d2a..b36ee4c 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1468,10 +1468,10 @@
     __system_properties_init; # apex
 
     # Used by libmemunreachable
-    malloc_backtrace; # apex
-    malloc_disable; # apex
-    malloc_enable; # apex
-    malloc_iterate; # apex
+    malloc_backtrace; # apex vndk
+    malloc_disable; # apex vndk
+    malloc_enable; # apex vndk
+    malloc_iterate; # apex vndk
 
     # Used by libandroid_net
     android_getaddrinfofornet; # apex