Reduce only-for-malloc area from 160k to 108k (#1794)

For non-SPIRAM ESP32s, this increases the size of the heap metadata from
12k to 16k, but may help for systems that have large Toit heaps and low
amounts of malloced memory.

src/third_party/dartino/gc_metadata.cc

@@ -43,10 +43,10 @@ void GcMetadata::set_up_singleton() {
   lowest_address_ = Utils::round_down(range_address, TOIT_PAGE_SIZE);
   uword size = Utils::round_up(range.size + range_address - lowest_address_, TOIT_PAGE_SIZE);
 #ifdef TOIT_FREERTOS
-  // Assume that the first 160k of memory can be used for C allocations, so we
+  // Assume that the first 108k of memory can be used for C allocations, so we
   // remove that from the area that needs to be covered by the heap metadata.
-  // This reduces the heap metadata from 24k or 28k to 12k or 16k.
-  const uword ONLY_FOR_MALLOC = 160 * KB;
+  // This reduces the heap metadata from 24k or 28k to 16k.
+  const uword ONLY_FOR_MALLOC = 108 * KB;
   const uword TWELVE_K_METADATA_LIMIT = 148 * KB;
   const uword SIXTEEN_K_METADATA_LIMIT = 200 * KB;
   if (!OS::use_spiram_for_metadata() && !OS::use_spiram_for_heap()) {