[gdbsupport] Add sequential_for_each

[gdbsupport] Add sequential_for_each

Add a sequential_for_each alongside the parallel_for_each, which can be used
as a drop-in replacement.

This can be useful when debugging multi-threading behaviour, and you want to
limit multi-threading in a fine-grained way.

Tested on x86_64-linux, by using it instead of the parallel_for_each in
dwarf2_build_psymtabs_hard.