Disable usage of mmap allocator in libbacktrace

Turns out it's just crazy slow on huge debug sections and it's also quite leaky.
We don't need to use it in a signal handler context, so let's just jettison the
mmap allocator

Author: alexcrichton

backtrace-sys/src/libbacktrace/configure

@@ -12335,7 +12335,7 @@ else
 
 _ACEOF
 if ac_fn_c_try_cpp "$LINENO"; then :
-  ALLOC_FILE=mmap.lo
+  ALLOC_FILE=alloc.lo
 else
   ALLOC_FILE=alloc.lo
 fi