gnu: mingw: Add x86_64 support.

gnu: mingw: Add x86_64 support.

This patch parameterizes previously hard-coded instances of
i686-w64-mingw32, adding support for x86_64-w64-mingw32.

* gnu/packages/mingw.scm (make-mingw-w64): New procedure.
(mingw-w64-i686, mingw-w64-x86_64): New variables.
(%mingw-triplet): Remove.
(mingw-w64): Update to point to 'mingw-w64-i686'.
* gnu/packages/cross-base.scm (cross-gcc): Use 'libc' keyword argument
if specified, instead of treating it as a boolean.
(native-libc): Return the correct mingw-w64 depending on machine
specified in target.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Add "x86_64-mingw".
* gnu/build/cross-toolchain.scm (set-cross-path/mingw): Replace
hardcoded 'i686-w64-mingw32' instances with 'target' keyword argument.
(cross-gcc-build-phases): Update accordingly; use 'target-mingw?'
implementation of target checking and add commentary.
* gnu/ci.scm (%cross-targets): Add "x86_64-w64-mingw32".