rust: Enable baremetal targets

Allow rust to build for baremetal targets by generating the proper target
triple, follow the format specified by rusts Triple [1], that is:

<arch>-<vendor>-<os>-<abi>

This is done automatically based on both TARGET_OS and TCLIBC.

For example, a riscv64 baremetal target triple would look like this:
riscv64gc-poky-none-elf

matching rusts own target triple for riscv64 according to platform-support [2]

[1] https://docs.rs/target-lexicon/latest/target_lexicon/struct.Triple.html
[2] https://doc.rust-lang.org/stable/rustc/platform-support.html



Signed-off-by: Alejandro Enedino Hernandez Samaniego <alejandro@enedino.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>