Always use size_t in dlmalloc's mallinfo

For some reason emscrpiten was overriding MALLINFO_FIELD_TYPE to be int
even though this defaults size_t and size_t is the document type in the
man page, and size_t allows for correct reporting of numbers larger than
2^32.

What is worse the `MALLINFO_FIELD_TYPE` override was only place when
`DLMALLOC_DEBUG` was defined which meant that `MALLINFO_FIELD_TYPE`
varied between `-sASSERTIONS=1` and `-sASSERTIONS=2` builds of dlmalloc.

This means that `-sMEMORY64 + -sASSERTIONS=2` builds of dlmalloc were
simply broken WRT `mallinfo` since the public definition of mallinfo
disagreed with the dlmalloc-internal version.

I verified that the follow tests fails without this change:

EMCC_CFLAGS=-sASSERTIONS=2 ./test/runner wasm64.test_mallinfo

I'm not updating the actual test code here since test coverage will be
coming in emscripten-core#23330.

This change was spit out from emscripten-core#23330.

system/include/compat/malloc.h

@@ -11,16 +11,16 @@ extern "C" {
    system/lib/dlmalloc.c.  */
 
 struct mallinfo {
-  int arena;    /* total space allocated from system */
-  int ordblks;  /* number of non-inuse chunks */
-  int smblks;   /* unused -- always zero */
-  int hblks;    /* number of mmapped regions */
-  int hblkhd;   /* total space in mmapped regions */
-  int usmblks;  /* unused -- always zero */
-  int fsmblks;  /* unused -- always zero */
-  int uordblks; /* total allocated space */
-  int fordblks; /* total non-inuse space */
-  int keepcost; /* top-most, releasable (via malloc_trim) space */
+  size_t arena;    /* total space allocated from system */
+  size_t ordblks;  /* number of non-inuse chunks */
+  size_t smblks;   /* unused -- always zero */
+  size_t hblks;    /* number of mmapped regions */
+  size_t hblkhd;   /* total space in mmapped regions */
+  size_t usmblks;  /* unused -- always zero */
+  size_t fsmblks;  /* unused -- always zero */
+  size_t uordblks; /* total allocated space */
+  size_t fordblks; /* total non-inuse space */
+  size_t keepcost; /* top-most, releasable (via malloc_trim) space */
 };
 
 /* The routines.  */

system/lib/dlmalloc.c

@@ -16,7 +16,6 @@
 #define ABORT __builtin_unreachable()
 /* allow malloc stats only in debug builds, which brings in stdio code. */
 #define NO_MALLOC_STATS 1
-#define MALLINFO_FIELD_TYPE int
 #endif
 /* XXX Emscripten Tracing API. This defines away the code if tracing is disabled. */
 #include <emscripten/trace.h>