[BOLT] Control aggregation mode output profile file format

In perf2bolt and `-aggregate-only` BOLT mode, the output profile file is written
in fdata format by default. Provide a knob `-profile-format=[fdata,yaml]` to
control the format.
Note that `-w` option still dumps in YAML format.

Reviewed By: #bolt, maksfb

Differential Revision: https://reviews.llvm.org/D133995