Cygwin: mmap: fix mapping beyond EOF on 64 bit

Commit 605bdcd410 enabled mapping beyond
EOF in 64 bit environments.  But the variable 'orig_len' did not get
rounded up to a multiple of 64K.  This rounding was done on 32 bit
only.  Fix this by rounding up orig_len on 64 bit, in the same place
where 'len' is rounded up.

Rounding up is needed to make sigbus_page_len a multiple of the
allocation granularity.

In addition, failing to round up could cause orig_len to be smaller
than len.  Since these are both unsigned values, the statement
'orig_len -= len' could then cause orig_len to be huge, and mmap would
fail with errno EFBIG.

I observed this failure while debugging the problem reported in

  https://sourceware.org/pipermail/cygwin/2020-July/245557.html.

The failure can be seen by running the test case in that report under
gdb or strace.

winsup/cygwin/mmap.cc

@@ -1144,6 +1144,7 @@ go_ahead:
 	 ends in, but there's nothing at all we can do about that. */
 #ifdef __x86_64__
       len = roundup2 (len, wincap.allocation_granularity ());
+      orig_len = roundup2 (orig_len, wincap.allocation_granularity ());
 #else
       len = roundup2 (len, wincap.is_wow64 () ? wincap.allocation_granularity ()
 					      : wincap.page_size ());