toolchain-scripts: Export meson settings for SDK builds

toolchain-scripts: Export meson settings for SDK builds

Create a new set of exports for the Meson `host_machine` cross settings.
This allows the target cross file to be created correctly from
meson.cross.template and aligns with meson.bbclass.

Note, one might think that HOST_OS and HOST_ARCH would be appropriate as
inputs here, aligning nicely with the Meson naming. That turns out to be
incorrect since the script is generated in a native/nativesdk build with
HOST_OS and HOST_ARCH set for the "build machine", not the "host
machine", using the Meson terminology. See
https://mesonbuild.com/Cross-compilation.html.

Fixes: [YOCTO #15485]
Signed-off-by: Tom Hochstein <tom.hochstein@oss.nxp.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>