[ELF] Cap parallel::strategy to 16 threads when --threads= is unspecified

When --threads= is unspecified, we set it to
`parallel::strategy.compute_thread_count()`, which uses
sched_getaffinity (Linux)/cpuset_getaffinity (FreeBSD)/std::thread::hardware_concurrency (others).
With extensive testing on many machines (many configurations from
{aarch64,x86-64} x {Linux,FreeBSD,Windows} x allocators(native,mimalloc,rpmalloc) combinations)
with varying workloads, we discovered that when the concurrency is larger than
16, the linking process is slower than using --threads=16 due to parallelism
overhead outweighs optimizations. This is particularly harmful for machines with
many cores or when the link job competes with other jobs.

Cap parallel::strategy when --threads= is unspecified.
For some workloads changing the concurrency from 8 to 16 has nearly no improvement.

--thinlto-jobs= is unchanged since ThinLTO backend compiles are embarrassingly
parallel.

Link: https://discourse.llvm.org/t/avoidable-overhead-from-threading-by-default/69160

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D147493

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