Profiling the profiler showed a lot of time spent allocating memory for the
frame-address strings, almost half the time of `StreamSamplesToJSON`!
Using an "Auto" string will prevent these allocations in most cases.
Also removed the `printf("0x%llx")`, instead just appending "0x" and an hex-
formatted number with `AppendInt()` (which handles 32- and 64-bit numbers
separately, so there is no more needs to do a double-cast to avoid sign
extensions -- There is still a double cast, but a no-op one to fix the type to
either `uint32_t` or `uint64_t`).
Using an Auto string for nearby frame labels as well.
Differential Revision: https://phabricator.services.mozilla.com/D45841
--HG--
extra : moz-landing-system : lando