gdb: define builtin long type to be 64 bits on amd64 Cygwin

On Windows x86-64 (when building with MinGW), the size of the "long"
type is 32 bits.  amd64_windows_init_abi therefore does:

    set_gdbarch_long_bit (gdbarch, 32);

This is also used when the chosen OS ABI is Cygwin, where the "long"
type is 64 bits.  GDB therefore gets sizeof(long) wrong when using the
builtin long type:

    $ ./gdb -nx --data-directory=data-directory -batch -ex "set architecture i386:x86-64" -ex "set osabi Cygwin" -ex "print sizeof(long)"
    The target architecture is assumed to be i386:x86-64
    $1 = 4

This patch makes GDB avoid setting the size of the long type to 32 bits
when using the Cygwin OS ABI.  it will inherit the value set in
amd64_init_abi.

With this patch, I get:

    $ ./gdb -nx --data-directory=data-directory -batch -ex "set architecture i386:x86-64" -ex "set osabi Cygwin" -ex "print sizeof(long)"
    The target architecture is assumed to be i386:x86-64
    $1 = 8

gdb/ChangeLog:

	PR gdb/21500
	* amd64-windows-tdep.c (amd64_windows_init_abi): Rename
	to...
	(amd64_windows_init_abi_common): ... this.  Don't set size of
	long type.
	(amd64_windows_init_abi): New function.
	(amd64_cygwin_init_abi): New function.
	(_initialize_amd64_windows_tdep): Use amd64_cygwin_init_abi for
	the Cygwin OS ABI.
	* i386-windows-tdep.c (_initialize_i386_windows_tdep): Clarify
	comment.