cross: Implement support for loading cross files from system paths

One thing that makes cross compiling with meson a pain is the need for
cross files. The problem is not with cross files themselves (they're
actually rather brilliant in that they allow for a much greater deal of
flexibility than autotools hardcoded paths approach) but that each user
needs to reimplement them themselves, when for most people what they
really want is a cross file that could be provided by their distro, all
they really want is the correct toolchain.

This patch is the first stop to making it easier for distros to ship
their own cross files (and for users to put their's somewhere safe so
they don't get `git clean`ed. It allows the cross files (on Linux and
*BSD) to be stored in home and system paths (~/.config/meson/cross,
/usr/share/meson/cross, and /usr/local/share/meson/cross), and to be
loaded by simply by specificying --cross-file.

With this patch meson will check the locations its always checked first,
(is cross file absolute, or is it relative to $PWD), then will check
~/.config/meson/cross, /usr/local/share/meson/cross,
/usr/share/meson/cross, (or $XDG_CONFIG_PATH and $XDG_DATA_DIRS) for the
files, raising an exception if it cannot find the specified cross file.

Fixes #2283

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