Add an option to reduce pagefault when available in Linux

This option instructs the kernel to synchronously load the entire mapped
region into active memory by specifying `MAP_POPULATE` in `mmap`. It will
cause read-ahead on that memory, and then the subsequent accesses to the
memory can proceed without page faults, improving some performance.

include/mimalloc.h

@@ -311,6 +311,9 @@ typedef enum mi_option_e {
   mi_option_page_reset,
   mi_option_abandoned_page_reset,
   mi_option_segment_reset,
+#if defined(__linux__)
+  mi_option_prefault,
+#endif
   mi_option_eager_commit_delay,
   mi_option_reset_delay,
   mi_option_use_numa_nodes,