Add a sampling profiler for machines without perf

benchmarks/sample.c runs a command and samples where its threads spend
user time (the task clock every 50 us, one perf_event_open event a CPU,
since the kernel maps no ring for an inherited per-task event), then
names the samples from the ELF symbol tables of the files the command had
mapped: a stripped library from its debug file by build id. On arm64 it
also takes the link register, so time in a library's leaf (memcpy,
strcmp) is shown again by its caller. The command's user and system time
head the report.

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