improve tools.cross_building to consider x86_64 -> x86 cross-compilation use-case

[SSE4]

often, x86_64 -> x86 cross-compilation could be considered the special case (if operation system is the same, of-course). it's because x86_64 systems may run x86 binaries, and usually there are so-called multilib compilers are available, so special preparations for cross-compilation aren't needed.
(NOTE: the opposite use-case x86 -> x86_64 isn't special, as x86 systems can't run x86_64 binaries)
there are many recipes which had to apply workarounds over tools.cross_building in order to satisfy their needs, e.g.:
https://github.com/conan-io/conan-center-index/blob/314ed91c6ecdbb6c21bba0eb3c438a6f3812060b/recipes/openssl/ALL/conanfile.py#L533
https://github.com/conan-io/conan-center-index/blob/9fd59cdef011bcbdcbcc3543cef9fa74714cb7e3/recipes/icu/all/conanfile.py#L145
https://github.com/bincrafters/conan-qt/blob/5093f8dc85ee01cb8b6d1baac2698c4b276cc9ce/conanfile.py#L518
it would be nice to provide an additional argument for tools.cross_building to return False for the the x86_64 -> x86 use-case (same OS)
proposal:

tools.cross_building(self.settings, native_x86=True)

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[Hopobcn]

Couldn't conan detect that the compiler is multilib to not require an extra paramter in tools.cross_building?

[SSE4]

@Hopobcn probably, it's also possible, as part of #5740