Use strwinerror in gdb/windows-nat.c

Use strwinerror in gdb/windows-nat.c

When working on windows-nat.c, it's useful to see an error message in
addition to the error number given by GetLastError.  This patch moves
strwinerror from gdbserver to gdbsupport, and then updates
windows-nat.c to use it.  A couple of minor changes to strwinerror
(constify the return type and use the ARRAY_SIZE macro) are also
included.