Flesh out a page size accessor in the new API.

Implement the old API in terms of the new one. This simplifies the
implementation on Windows which can now re-use the self_process's once
initialization.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171330 91177308-0d34-0410-b5e6-96231b3b80d8

unittests/Support/ProcessTest.cpp

@@ -28,6 +28,8 @@ TEST(ProcessTest, SelfProcess) {
 #elif defined(LLVM_ON_WIN32)
   EXPECT_EQ(GetCurrentProcess(), process::get_self()->get_id());
 #endif
+
+  EXPECT_LT(1u, process::get_self()->page_size());
 }
 
 } // end anonymous namespace