dlltool: Fatal error: no compiled in support for x86_64

Question

I'm trying to build a 64-bit version of libpython27.a from python27.dll on Windows 7 x64.

Following these instructions, I tried:

E:\temp>gendef python27.dll
 * [python27.dll] Found PE+ image

E:\temp>dlltool --as-flags=--64 -m i386:x86-64 -k --output-lib libpython27.a --i
nput-def python27.def
Assembler messages:
Fatal error: no compiled in support for x86_64
dlltool: as exited with status 1
dlltool: failed to open temporary head file: dgtnh.o: No such file or directory

What can explain Fatal error: no compiled in support for x86_64?

I see:

E:\temp>dlltool -h
Usage dlltool <option(s)> <object-file(s)>
   -m --machine <machine>    Create as DLL for <machine>.  [default: i386:x86-64]
        possible <machine>: arm[_interwork], i386, mcore[-elf]{-le|-be}, ppc, thumb
   -e --output-exp <outname> Generate an export file.

Does the issue come from the fact possible <machine> does not contain x86_64, and if so where to find a dlltool.exe that supports x86_64?