When single-threaded mode is selected, all instances of the keyword
`LIBC_THREAD_LOCAL` are stubbed out, similar to how it currently works
on the GPU. This allows baremetal builds to avoid using thread_local.
However, libcxx uses shared headers, so we need to be careful there.
Thankfully, there is already an option to disable multithreading in
libcxx, so a flag is added such that single-threaded mode is propagated
down to libc.
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