Add size-gated hugepage optimization to system allocator

This patch adds optional hugepage optimization by applying madvise(MADV_HUGEPAGE)
to allocations that are large enough to benefit from transparent hugepages.

Current behavior:
- Uses MADV_NOHUGEPAGE for infrequent access patterns, MADV_HUGEPAGE only for memory collapsing
- No proactive hugepages for large allocations

Enhanced behavior:
- Adds MADV_HUGEPAGE for allocations >= kHugePageSize (2MB)
- Only applies to allocations that can actually benefit from hugepages
- Avoids kernel overhead on small allocations

The size threshold ensures we only hint hugepage usage for allocations
that can actually be backed by hugepages, avoiding unnecessary kernel
overhead for smaller allocations.

Author: hsane-dev

tcmalloc/internal/system_allocator.h

@@ -377,6 +377,15 @@ SystemAllocator<Topology, NormalPartitions>::MmapRegion::Alloc(
     // This is only advisory, so ignore the error.
     ErrnoRestorer errno_restorer;
     (void)madvise(result_ptr, actual_size, MADV_NOHUGEPAGE);
+  } else {
+    // Opt-in to transparent hugepages for large allocations when system is 
+    // configured for transparent_hugepage=madvise. This can improve memory
+    // performance by reducing TLB pressure.
+    if (actual_size >= kHugePageSize) {
+      // This is only advisory, so ignore the error.
+      ErrnoRestorer errno_restorer;
+      (void)madvise(result_ptr, actual_size, MADV_HUGEPAGE);
+    }
   }
   free_size_ -= actual_size;
   return {result_ptr, actual_size};