Add support for modifying decay timer.

Add the mallopt function, and only a single option so far.

Bug: 36401135

Test: Built and booted bullhead.
Test: Ran jemalloc unit tests.
Test: Ran bionic unit tests.
Test: Ran a test that allocated and free'd a large piece of memory,
Test: and verified that after changing the parameter, the PSS
Test: sticks around (decay timer set to 1), the PSS is purged (decay
Test: timer set to 0).
Change-Id: I6927929b0c539c1023d34772d9e26bb6a8a45877
diff --git a/libc/bionic/malloc_common.cpp b/libc/bionic/malloc_common.cpp
index 18c998d..1f201d1 100644
--- a/libc/bionic/malloc_common.cpp
+++ b/libc/bionic/malloc_common.cpp
@@ -68,6 +68,7 @@
     Malloc(iterate),
     Malloc(malloc_disable),
     Malloc(malloc_enable),
+    Malloc(mallopt),
   };
 
 // In a VM process, this is set to 1 after fork()ing out of zygote.
@@ -101,6 +102,14 @@
   return Malloc(mallinfo)();
 }
 
+extern "C" int mallopt(int param, int value) {
+  auto _mallopt = __libc_globals->malloc_dispatch.mallopt;
+  if (__predict_false(_mallopt != nullptr)) {
+    return _mallopt(param, value);
+  }
+  return Malloc(mallopt)(param, value);
+}
+
 extern "C" void* malloc(size_t bytes) {
   auto _malloc = __libc_globals->malloc_dispatch.malloc;
   if (__predict_false(_malloc != nullptr)) {
@@ -247,6 +256,10 @@
                                           prefix, "mallinfo")) {
     return false;
   }
+  if (!InitMallocFunction<MallocMallopt>(malloc_impl_handler, &table->mallopt,
+                                         prefix, "mallopt")) {
+    return false;
+  }
   if (!InitMallocFunction<MallocMalloc>(malloc_impl_handler, &table->malloc,
                                         prefix, "malloc")) {
     return false;