Add setTargets to AbstractGccCompatibleToolchain to allow resetting default toolchain targets.

[JaciBrunning]

Context

This pull request is designed to allow users to reset the default platforms assigned to Gcc and Clang toolchains. This is a new version of #3061 per @ghale 's suggestion.

setTargets(String) and setTargets(String, Action) have been added to GccCompatibleToolchain.

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[ghale]

Thanks for making those changes, @JacisNonsense! Looks good.

I think there are two improvements we could make here:

1. We might change setTargets(String) to accept multiple targets: setTargets(String...). Optionally, we could add a setTargets(List<String>, Action) that allowed one to set multiple targets, and apply the same action to each, but I don't think this is necessary as it can be accomplished via eachPlatform(Action).
2. We should have some integration test coverage for this in GccToolChainCustomisationIntegrationTest. This would create a gcc toolchain (with x86, x64 targets), a custom toolchain that sets the targets to a custom platform and then use eachPlatform() to configure a custom compiler that sets the language to french (see

   gradle/subprojects/platform-native/src/integTest/groovy/org/gradle/nativeplatform/toolchain/GccToolChainCustomisationIntegrationTest.groovy

   Line 137 in fe5e133

   def "can configure platform specific executables"() {

   for an example of how you can do this simply). Your component would target all three platforms. Then assemble and verify that the x86, x64 outputs are in english (built by gcc), and the custom platform output is in french (built by custom toolchain).

[JaciBrunning]

@ghale I've added the changes mentioned, changing to setTargets(String...) and adding integration test. Integration test will have to be checked by CI since I don't have a machine available to integration test at this time

[ghale]

Thanks @JacisNonsense! I made a few minor changes to get the integration test working and merged it.