infer linker flavor by linker name if it's sufficiently specific

[usamoi]

Fix: rustc does not infer llvm-bitcode-linker uses llbc linker flavor if targeting nvptx64-nvidia-cuda.

[rustbot]

r? @davidtwco

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[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[lqd]

cc @petrochenkov