Previously, there was an option ZIG_PREFER_LLVM_CONFIG which would
override the default of not using llvm-config when cross compiling.
That option is now removed in favor of the more powerful
ZIG_USE_LLVM_CONFIG which defaults to OFF for cross compiling and ON for
native compilation. The option overrides the default.
This will be used in zig-bootstrap to improve support for native builds.
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