[clang] [llvm] runtimes: Pass CMAKE_SYSTEM_NAME based on target triple (PR #203504)

[Matt Arsenault via cfe-commits]

arsenm wrote:

Interesting:
```
Unexpectedly Passed Tests (12):
    llvm-libc++-llvm-libc.cfg.in :: std/containers/container.adaptors/container.adaptors.format/format.functions.format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/containers/container.adaptors/container.adaptors.format/format.functions.vformat.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtmap/format.functions.format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtmap/format.functions.vformat.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtset/format.functions.format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtset/format.functions.vformat.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtstr/format.functions.format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtstr/format.functions.vformat.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtstr/format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.fmtstr/parse.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.formatter/format.functions.format.pass.cpp
    llvm-libc++-llvm-libc.cfg.in :: std/utilities/format/format.range/format.range.formatter/format.functions.vformat.pass.cpp
```

https://github.com/llvm/llvm-project/pull/203504