rustc: Allow targets to specify SIMD args are by-val

rustc: Allow targets to specify SIMD args are by-val

The upcoming SIMD support in the wasm target is unique from the other
platforms where it's either unconditionally available or not available,
there's no halfway where a subsection of the program can use it but no
other parts of the program can use it. In this world it's valid for wasm
SIMD args to always be passed by value and there's no need to pass them
by reference.

This commit adds a new custom target specification option
`simd_types_indirect` which defaults to `true`, but the wasm backend
disables this and sets it to `false`.