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/malloc_debug/malloc_debug.cpp b/libc/malloc_debug/malloc_debug.cpp
index addb5d4..014d385 100644
--- a/libc/malloc_debug/malloc_debug.cpp
+++ b/libc/malloc_debug/malloc_debug.cpp
@@ -77,6 +77,7 @@
 void* debug_realloc(void* pointer, size_t bytes);
 void* debug_calloc(size_t nmemb, size_t bytes);
 struct mallinfo debug_mallinfo();
+int debug_mallopt(int param, int value);
 int debug_posix_memalign(void** memptr, size_t alignment, size_t size);
 int debug_iterate(uintptr_t base, size_t size,
     void (*callback)(uintptr_t base, size_t size, void* arg), void* arg);
@@ -639,6 +640,10 @@
   return g_dispatch->mallinfo();
 }
 
+int debug_mallopt(int param, int value) {
+  return g_dispatch->mallopt(param, value);
+}
+
 int debug_posix_memalign(void** memptr, size_t alignment, size_t size) {
   if (DebugCallsDisabled()) {
     return g_dispatch->posix_memalign(memptr, alignment, size);