The issue is uncovered by #47698: for assembly files, -triple= specifies the
full target triple while -arch= merely sets the architecture part of the default
target triple, leaving a target triple which may not make sense, e.g.
riscv64-apple-darwin.
Therefore, -arch= is error-prone and not recommended for tests. The issue has
been benign as we recognize $unknown-apple-darwin as ELF instead of rejecting it
outrightly.
Due to the nature of the issue, we don't see the issue in tests using
architectures that any of Mach-O/COFF/XCOFF supports.
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