lang/rust-bootstrap: Unbreak aarch64 bootstrap

Something goes wrong when it is built with LLVM > 9. Build it with
llvm90 for the time being.

rustc -Cllvm-args="..." with: Unknown command line argument '-generate-arange-section'. Try: 'rustc -Cllvm-args="..." with --help'
rustc -Cllvm-args="..." with: Unknown command line argument '-mergefunc-use-aliases'. Try: 'rustc -Cllvm-args="..." with --help'
rustc -Cllvm-args="..." with: Unknown command line argument '-preserve-alignment-assumptions-during-inlining=false'. Try: 'rustc -Cllvm-args="..." with --help'

or when built with llvm.assertions=true:

Assertion failed: (hasOptions() && "No options specified!"), function ParseCommandLineOptions, file src/llvm-project/llvm/lib/Support/CommandLine.cpp, line 1344.