[trace][intel pt] Add a nice parser for the trace size

Thanks to rnofenko@fb.com for coming up with these changes.

This diff adds support for passing units in the trace size inputs. For example,
it's now possible to specify 64KB as the trace size, instead of the
problematic 65536. This makes the user experience a bit friendlier.

Differential Revision: https://reviews.llvm.org/D129613