[feature] MesonToolchain: improve cross-compilation workflow on Apple systems

[SSE4]

unlike CMake, meson is dumb and doesn't attempt to guess a path to the compiler in case of cross-compiling mode.
it has to be explicitly specified within cross-file

we probably can deduce required things in the conan client, but it's kinda auto-magic...
OTH it makes recipes easier

we may try to populate the contents of the cross-file similar to the examples meson provides:
https://github.com/mesonbuild/meson/blob/master/cross/iphone.txt
https://github.com/mesonbuild/meson/blob/master/cross/tvos.txt

see also:
conan-io/conan-center-index#7360
conan-io/conan-center-index#6678

/cc @ericLemanissier @madebr @SpaceIm

• I've read the CONTRIBUTING guide.

[lasote]

I would say that the Toolchain is supposed to do exactly that, gather the settings and write something the build system understands to represent the configuration.
Said that, I would say we should consider also adjusting the compiler as I commented here: #9922 (comment)
@memsharded

[lasote]

I'm assigning as look into for the next release and let's see if we can finally do it for this release, wait to the next or...let's see