Support repeated flags in platform mappings and platform-based flags

[katre]

This was discovered while implementing platform-based flags, #19409. Since this uses the same BuildOptions.applyParsingResult method as platform mappings, the same issue applies to both.

A quick definition: a repeatable flag is a flag where repeated uses accumulate, instead of overwriting. This corresponds to using @Option.allowMultiple in a native flag or defining a Starlark flag using config.string_list(repeatable = True). Commonly used flags that use this are --features and --copt.

Currently, any repeatable flags used in platform mapping or platform based flags will overwrite any previous values. If any platforms set the --features flag, for example, this will remove any values set on the command line.

Ideally we would either intelligently merge these, or at least warn users that it happens.

There are example tests that show the error: #22386 (see errors: https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/21626).

This is a tracking issue for dealing with this.