Cygwin: shmat: use mmap allocator strategy on 64 bit

This avoids collisions of shmat maps with Windows own datastructures
when allocating top-down.

This patch moves the mmap_allocator class definition into its
own files and just uses it from mmap and shmat.

winsup/cygwin/mmap_alloc.h

@@ -0,0 +1,21 @@
+#ifdef __x86_64__
+
+/* The memory region used for memory maps */
+#define MMAP_STORAGE_LOW	0x001000000000L	/* Leave 32 Gigs for heap. */
+/* Up to Win 8 only supporting 44 bit address space, starting with Win 8.1
+   48 bit address space. */
+#define MMAP_STORAGE_HIGH	wincap.mmap_storage_high ()
+
+class mmap_allocator
+{
+  caddr_t mmap_current_low;
+
+public:
+  mmap_allocator () : mmap_current_low ((caddr_t) MMAP_STORAGE_HIGH) {}
+
+  PVOID alloc (PVOID in_addr, SIZE_T in_size, bool fixed);
+};
+
+extern mmap_allocator mmap_alloc;
+
+#endif