Support jemalloc compiled with `--with-jemalloc-prefix` (#5521)

Summary:
Previously, if the jemalloc was built with nonempty string for
`--with-jemalloc-prefix`, then `HasJemalloc()` would return false on
Linux, so jemalloc would not be used at runtime. On Mac, it would cause
a linker failure due to no definitions found for the weak functions
declared in "port/jemalloc_helper.h". This should be a rare problem
because (1) on Linux the default `--with-jemalloc-prefix` value is the
empty string, and (2) Homebrew's build explicitly sets
`--with-jemalloc-prefix` to the empty string.

However, there are cases where `--with-jemalloc-prefix` is nonempty.
For example, when building jemalloc from source on Mac, the default
setting is `--with-jemalloc-prefix=je_`. Such jemalloc builds should be
usable by RocksDB.

The fix is simple. Defining `JEMALLOC_MANGLE` before including
"jemalloc.h" causes it to define unprefixed symbols that are aliases for
each of the prefixed symbols. Thanks to benesch for figuring this out
and explaining it to me.

Fixes https://github.com/facebook/rocksdb/issues/1462.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5521

Test Plan:
build jemalloc with prefixed symbols:

```
$ ./configure --with-jemalloc-prefix=lol
$ make
```

compile rocksdb against it:

```
$ WITH_JEMALLOC_FLAG=1 JEMALLOC=1 EXTRA_LDFLAGS="-L/home/andrew/jemalloc/lib/" EXTRA_CXXFLAGS="-I/home/andrew/jemalloc/include/" make -j12 ./db_bench
```

run db_bench and verify jemalloc actually used:

```
$ ./db_bench -benchmarks=fillrandom -statistics=true -dump_malloc_stats=true -stats_dump_period_sec=1
$ grep jemalloc /tmp/rocksdbtest-1000/dbbench/LOG
2019/06/29-12:20:52.088658 7fc5fb7f6700 [_impl/db_impl.cc:837] ___ Begin jemalloc statistics ___
...
```

Differential Revision: D16092758

fbshipit-source-id: c2c358346190ed62ceb2a3547a6c4c180b12f7c4

db/malloc_stats.cc

@@ -20,10 +20,6 @@ namespace rocksdb {
 
 #ifdef ROCKSDB_JEMALLOC
 
-#ifdef JEMALLOC_NO_RENAME
-#define malloc_stats_print je_malloc_stats_print
-#endif
-
 typedef struct {
   char* cur;
   char* end;

port/jemalloc_helper.h

@@ -9,6 +9,7 @@
 #ifdef __FreeBSD__
 #include <malloc_np.h>
 #else
+#define JEMALLOC_MANGLE
 #include <jemalloc/jemalloc.h>
 #endif