rust: Introduce arch_to_rust_arch()

rust: Introduce arch_to_rust_arch()

On modern Power systems `uname -m` yields 'ppc64le' while the toolchain
knows the architecture as 'powerpc64le'. Provide a mapping from one to
the other to integrate with the existing architecture configuration
flags.

arch_to_rust_arch() only exists to map the OE *_ARCH variables before
any further processing, unlike arch_to_rust_target_arch() which is
specific to the internal triple handling of rust.

On Linux ppc64le systems the changes give the following config:

```
$ cat ./tmp/work/ppc64le-linux/rust-native/1.58.0-r0/targets/ppc64le-linux.json
{
    "llvm-target": "powerpc64le-unknown-linux-gnu",
    "data-layout": "e-m:e-i64:64-n32:64-v256:256:256-v512:512:512",
    "max-atomic-width": 64,
    "target-pointer-width": "64",
    "target-c-int-width": "64",
    "target-endian": "little",
    "arch": "powerpc64",
    "os": "linux",
    "env": "gnu",
    "vendor": "unknown",
    "target-family": "unix",
    "linker": "gcc",
    "cpu": "generic",
    "dynamic-linking": true,
    "executables": true,
    "linker-is-gnu": true,
    "linker-flavor": "gcc",
    "has-rpath": true,
    "has-elf-tls": true,
    "position-independent-executables": true,
    "panic-strategy": "unwind"
}
```

Change-Id: Ief0c01189185d7d4da31d307270bec4e1de674ca
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>