"jsallsyms", for work in progress on perf+JS
Work around Linux/arm bug that drops usermode PC by using PERF_SAMPLE_IP.

FIXME: this duplicates the topmost frame if ever run on a platform
without this "feature", or if/when the kernel is fixed.

Unavoidable deficiency: this won't preserve the topmost user frame if
the CPU is in kernel mode for the sample.