Make vendorSystickConfig only required for ARM

[kurtjd]

Adds Target enum to Config to allow different behavior during parsing depending on CPU architecture. Specifically, for now this is used to make vendorSystickConfig optional (defaulting to false) for non-ARM targets.

This reuses the Target enum from svd2rust and ideally this could then be used in svd2rust, but I didn't want to copy over the serde cfgs since there isn't a serde feature for svd-parser so may just have to have this minor code duplication in the two crates.

Resolves #296

[kurtjd]

CI failure seems to be unrelated to my changes. Looks like a dep needs a greater rustc version than is being used in CI?

[burrbull]

try to add indexmap = "=2.11.4" to svd-encoder Cargo.toml

[kurtjd]

try to add indexmap = "=2.11.4" to svd-encoder Cargo.toml

Thanks, looks like it did the trick at least running cargo +1.70.0 check locally. Pushed another commit with that change.

[burrbull]

cc @Emilgardis