Propagate linker_type correctly in the system_cxx_toolchain

[Nehliin]

Previously the system cxx toolchain would hard code the linker type based off the os. This works fine in most cases but breaks windows gnu builds for example. The linker_type is already correctly provided by the CxxToolsInfo type but ignored. This pr uses the linker_type from there instead and adds a select to the path clang tools to separate darwin from gnu targets.

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