fix folly:memory build on macOS

Summary:
Don't assume a jemalloc target exists (even if it would be a no-op, as
in on macOS). Instead compute the correct set of external_deps in
jemalloc.bzl.

This fixes the folly build on mode/mac after D14179337.

Reviewed By: andrewjcg

Differential Revision: D14235815

fbshipit-source-id: d1bfbb91897de9557524bbb4b5736ca7b03cbf9b

folly/memory/Malloc.h

@@ -29,7 +29,7 @@
  */
 #if (defined(USE_JEMALLOC) || defined(FOLLY_USE_JEMALLOC)) && !FOLLY_SANITIZE
 // We have JEMalloc, so use it.
-#include <jemalloc/jemalloc.h>
+#include <jemalloc/jemalloc.h> // @manual
 #else
 #ifndef MALLOCX_LG_ALIGN
 #define MALLOCX_LG_ALIGN(la) (la)

folly/portability/Malloc.h

@@ -24,7 +24,7 @@
 #if (defined(USE_JEMALLOC) || defined(FOLLY_USE_JEMALLOC)) && !FOLLY_SANITIZE
 // JEMalloc provides it's own implementation of
 // malloc_usable_size, and that's what we should be using.
-#include <jemalloc/jemalloc.h>
+#include <jemalloc/jemalloc.h> // @manual
 #else
 #ifndef __APPLE__
 #include <malloc.h>