Since 37194a325d llvmPackages*.bintools is a bintools-wrapper. Thus it
only contains a wrapper for `as` and `ld`. This change makes sense, but
causes regressions like this one. Since the buildStdenv uses the llvm
bintool set including lld as a linker we can use the
cc.bintools.bintools derivation to get all the tools we need.
Technically we wouldn't need to set absolute paths as all tools are also
added to PATH, but it doesn't hurt either.
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