configure, meson: detect Rust toolchain

configure, meson: detect Rust toolchain

Include the correct path and arguments to rustc in the native
and cross files (native compilation is needed for procedural
macros).

Based on the host architecture and OS, the compiler and optionally the argument
to --cpu, the Rust target triple can be detected automatically for either a
native or a cross compiler.

In general, it is only a matter of translating the architecture and OS, and
adding a machine to form the triple, but there are some special cases (e.g.
detecting soft vs. hard floating point on ARM) and some inconsistencies.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Link: https://lore.kernel.org/r/207d2640b32d511e9c27478ce3192f5bb0bf3169.1727961605.git.manos.pitsidianakis@linaro.org
[Leave disabled by default until CI covers the Rust code on supported
 distros. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>