package/rustc: rust >= 1.73.0 needs cmake >= 3.20

The bump to rust 1.74.1 [1] introduced a regression on host using
cmake < 3.20 since the llvm libraries used in rust compiler was
bumpted to llvm 17 [2] and now requires cmake >= 3.20 [3].

Select BR2_HOST_CMAKE_AT_LEAST_3_20 to build host-cmake when needed.

[1] https://gitlab.com/buildroot.org/buildroot/-/commit/05392a5eae61c2855bc8d94f5bf9677ebbc5462a
[2] https://github.com/rust-lang/rust/commit/8c1c7d37b29d72bad1f218798d121074918e9616
[3] https://github.com/llvm/llvm-project/commit/cbaa3597aaf6273e66b3f445ed36a6458143fe6a

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/5880448635

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Antoine Coutant <antoine.coutant@smile.fr>
Reviewed-by: Antoine Coutant <antoine.coutant@smile.fr>
Signed-off-by: Romain Naour <romain.naour@smile.fr>

package/rustc/Config.in.host

@@ -149,6 +149,8 @@ config BR2_PACKAGE_HOST_RUST
 	depends on BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
 	# triggers ICE on trunc_int_for_mode, at explow.c:56
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_5 || !BR2_aarch64
+	# rust uses llvm >= 17 since 1.73.0
+	select BR2_HOST_CMAKE_AT_LEAST_3_20
 	help
 	  This package will build the compiler for the host as well as
 	  two flavors of the standard library: one for the host, another