Many APIs in the "os" class (e.g, os::vm_page_size()) return information about the OS that's collected during VM bootstrap and never changes afterwards.

Add a new OSInfo API to access such information efficiently, and simplify the per-OS implementation:

class OSInfo : AllStatic {
  static int _vm_page_size;
  static int _vm_allocation_granularity;
  [... more ...]

public:
  // Returns the byte size of a virtual memory page
  static int vm_page_size() { return _vm_page_size; }

  // Returns the size in bytes of memory blocks which can be allocated.
  static int vm_allocation_granularity() { return _vm_allocation_granularity; }

   static void set_vm_page_size(int n) {
    assert(_vm_page_size < 0, "init only once");
    assert(n > 0, "sanity");
    _vm_page_size = n;
  }

  static void set_vm_allocation_granularity(int n) {
    assert(_vm_allocation_granularity < 0, "init only once");
    assert(n > 0, "sanity");
    _vm_allocation_granularity = n;
  }

   [... more ...]