Compilation: fix compiler_rt and ubsan_rt strategy logic

It doesn't really make sense for `target_util.canBuildLibCompilerRt`
(and its ubsan-rt friend) to take in `use_llvm`, because the caller
doesn't control that: they're just going to queue a sub-compilation for
the runtime. The only exception to that is the ZCU strategy, where we
effectively embed `_ = @import("compiler_rt")` into the Zig compilation:
there, the question does matter. Rather than trying to do multiple weird
calls to model this, just have `canBuildLibCompilerRt` return not just a
boolean, but also differentiate the self-hosted backend being capable of
building the library vs only LLVM being capable. Logic in `Compilation`
uses that difference to decide whether to use the ZCU strategy, and also
to disable the library if the compiler does not support LLVM and it is
required.

Also, remove a redundant check later on, when actually queuing jobs.
We've already checked that we can build `compiler_rt`, and
`compiler_rt_strat` is set accordingly. I'm guessing this was there to
work around a bug I saw in the old strategy assignment, where support
was ignored in some cases.

Resolves: #24623

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