rpmalloc: Switch back to using 2MB heap areas instead of 4MB.

There seem to be some cases that severely fragment rpmalloc's heap,
wasting large amounts of memory (in #15264, some 750+MB it appears
are wasted this way.) This is a stop-gap measure to cut the wastage
in half (or more) until a proper solution is implemented.

src/system/libroot/posix/rpmalloc/rpmalloc.cpp

@@ -61,9 +61,13 @@
 #define DISABLE_UNMAP             0
 #endif
 #ifndef DEFAULT_SPAN_MAP_COUNT
+#ifdef __HAIKU__
+#define DEFAULT_SPAN_MAP_COUNT    32
+#else
 //! Default number of spans to map in call to map more virtual memory (default values yield 4MiB here)
 #define DEFAULT_SPAN_MAP_COUNT    64
 #endif
+#endif
 
 #if ENABLE_THREAD_CACHE
 #ifndef ENABLE_UNLIMITED_CACHE