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/private/bionic_malloc_dispatch.h b/libc/private/bionic_malloc_dispatch.h
index 02a092f..cdae466 100644
--- a/libc/private/bionic_malloc_dispatch.h
+++ b/libc/private/bionic_malloc_dispatch.h
@@ -45,6 +45,7 @@
 typedef int (*MallocIterate)(uintptr_t, size_t, void (*)(uintptr_t, size_t, void*), void*);
 typedef void (*MallocMallocDisable)();
 typedef void (*MallocMallocEnable)();
+typedef int (*MallocMallopt)(int, int);
 
 #if defined(HAVE_DEPRECATED_MALLOC_FUNCS)
 typedef void* (*MallocPvalloc)(size_t);
@@ -69,6 +70,7 @@
   MallocIterate iterate;
   MallocMallocDisable malloc_disable;
   MallocMallocEnable malloc_enable;
+  MallocMallopt mallopt;
 } __attribute__((aligned(32)));
 
 #endif