trace: use dynamic string buffer in make_flamegraph()

trace: use dynamic string buffer in make_flamegraph()

The str[] buffer declared in make_flamegraph() is used to hold strings
representing the full call-stacks recorded in traces. The size of this
buffer is currently 500 characters and this works well for the documented
examples.

However, it is possible to exhaust this buffer when processing traces
captured when running the UEFI shell on aarch64 sandbox for example.
Indeed, the maximum length needed for such traces can reach 780 characters.

As it is difficult to evaluate the maximum size that would ever be needed
for all the possible traces, let's use a dynamically allocated `abuf'
instead, which we reallocate when needed.

This fixes the following error:

  String too short (500 chars)

While at it, fix a few typos in strings and comments.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Michal Simek <michal.simek@amd.com>