Fix huge_pages on Windows

Since Windows 10 1703, it is additionally necessary to pass a flag
called FILE_MAP_LARGE_PAGES to MapViewOfFile() to enable large pages at
map time.  This flag is ignored on older versions of Windows, where
large pages should still be able to work properly without setting it.
Note that the flag would be set only for binaries that knew about it at
compile-time, which should be more or less all the Windows environments
these days.

Since 495ed0e, Windows 10 is the minimum version of Windows supported by
Postgres, making this change easy to reason about on HEAD.  Per
discussion, no backpatch is done for the moment.

Reported-by: Okano Naoki
Author: Thomas Munro
Reviewed-by: Tom Lane, Michael Paquier, Julien Rouhaud
Discussion: https://postgr.es/m/17448-0a96583a67edb1f7@postgresql.org

src/backend/port/win32_shmem.c

@@ -218,6 +218,7 @@ PGSharedMemoryCreate(Size size,
 	SIZE_T		largePageSize = 0;
 	Size		orig_size = size;
 	DWORD		flProtect = PAGE_READWRITE;
+	DWORD		desiredAccess;
 
 	ShmemProtectiveRegion = VirtualAlloc(NULL, PROTECTIVE_REGION_SIZE,
 										 MEM_RESERVE, PAGE_NOACCESS);
@@ -355,12 +356,19 @@ retry:
 	if (!CloseHandle(hmap))
 		elog(LOG, "could not close handle to shared memory: error code %lu", GetLastError());
 
+	desiredAccess = FILE_MAP_WRITE | FILE_MAP_READ;
+
+#ifdef FILE_MAP_LARGE_PAGES
+	/* Set large pages if wanted. */
+	if ((flProtect & SEC_LARGE_PAGES) != 0)
+		desiredAccess |= FILE_MAP_LARGE_PAGES;
+#endif
 
 	/*
 	 * Get a pointer to the new shared memory segment. Map the whole segment
 	 * at once, and let the system decide on the initial address.
 	 */
-	memAddress = MapViewOfFileEx(hmap2, FILE_MAP_WRITE | FILE_MAP_READ, 0, 0, 0, NULL);
+	memAddress = MapViewOfFileEx(hmap2, desiredAccess, 0, 0, 0, NULL);
 	if (!memAddress)
 		ereport(FATAL,
 				(errmsg("could not create shared memory segment: error code %lu", GetLastError()),