gdb/record: extract the PC to record_full_instruction

This commit makes it so the PC is not saved as part of the
record_full_instruction effects, but rather gets a special location.
That is because a couple of commands would really benefit from it being
easy to find the PC (especially ones from record-btrace that's haven't
been implemented to record-full yet, such as the ones in PR
record/18059), while also possibly allowing for one fewer resizing of
the effect vector (and saving an entire byte in the process).

This commit also refactored record_full_read_entry_from_bfd and
record_full_write_entry_to_bfd, to make them methods of
record_full_reg_entry and record_full_mem_entry, and also creates
similar methods for record_full_entry and record_full_instruction.
These could be turned into constructors in a future step of
c++ification, but it felt like too much change for a single commit.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-By: Christina Schimpe <christina.schimpe@intel.com>
Approved-By: Guinevere Larsen <guinevere@redhat.com>