s390x, etc: Support leonbe?

Thanks for filing a feature request! Please fill out the TODOs below.

#### Feature

We want to see wasmtime use/support the leonbe addressing convention as implemented in wasm2c instead of the current lemulation addressing convention.

#### Benefit

Consistency with another wasm implementation. Presumably easier for the JIT to optimize. Allegedly improved performance on older BE platforms without fused byteswap opcodes. (This could all be benchmarked.)

#### Implementation

It shouldn't be too hard, mostly changing memory instructions and some other stuff.

#### Alternatives

The main alternative is to get rid of wasm2c's leonbe and make it use lemulation, or maybe even go the extra length and make it portable to mixed-endian architectures. Unfortunately the compiler isn't allowed to entirely eliminate byteswaps with that approach.