It appears that the RUNTIMES build prefers the x86-64-unknown-linux-gnu
triple notation for the host. This fixes runtime / test breakages when
compiler-rt is used as the CLANG_DEFAULT_RTLIB.
Sometimes it might be beneficial to spawn more thread blocks instead of
reusing existing for multiple loop iterations.
**Alternatives considered:**
Make `DefaultNumBlocks` settable via an environment variable.
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Co-authored-by: Joseph Huber <huberjn@outlook.com>