Provide method to dump backtrace heap data.

For non-zygote spawned processes, we might want to dump the backtrace
data. Provide a method to send a signal to a process and then dump the
data to a file.

Adds a method to dump the backtrace data on exit.

Update documentation and explain format of heap dump data.

Test: Ran unit tests, enabled new options and used them.
Change-Id: Ie2fa706694160731afe02c1382b037d06df1d069
diff --git a/libc/malloc_debug/exported64.map b/libc/malloc_debug/exported64.map
index ec9d840..94104b0 100644
--- a/libc/malloc_debug/exported64.map
+++ b/libc/malloc_debug/exported64.map
@@ -1,6 +1,7 @@
 LIBC_MALLOC_DEBUG {
   global:
     debug_calloc;
+    debug_dump_heap;
     debug_finalize;
     debug_free;
     debug_free_malloc_leak_info;