gnu: cross-base: Don't specify mingw --with-newlib

Previous to this commit, we added a --with-newlib configure flag to
cross-gcc when cross-newlib?, but cross-newlib? is true only when
target-mingw?.

It turns out that specifying --with-newlib disables the
GLIBCXX_CROSSCONFIG check, which is used to detect
_GLIBCXX_HAVE__WFOPEN, which is required in C++17 std::filesystem for
mingw-w64 systems.

Additional context: In gnu/packages/embedded.scm, --with-newlib is
specified explicitly when we're actually using newlib, which seems like
the correct way of handling it.

Situation in other distros:
- Debian's gcc-mingw-w64 doesn't specify --with-newlib
- Fedora's mingw64-gcc-c++ explicitly specifies --without-newlib

Chesterton's fence: Chatting with janneke, who originally added this
mechanism, reveals that this flag is not only no longer required, but
also that removing it doesn't break his guile-mingw builds. See IRC logs
of #guix for 2022-02-15.

* gnu/packages/cross-base.scm (cross-gcc-arguments): Don't check for and
specify --with-newlib.
(cross-libc): Check for mingw and use mingw-w64 directly.
(cross-newlib?): Remove, unexport.
(native-libc): Remove.

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