Revert "Do not overwrite so_list's so_name in solib_map_sections"

Revert "Do not overwrite so_list's so_name in solib_map_sections"

This reverts commit 07293be44859c607a36c313e51bec2dcdcd3c243, as it
causes an unintended change of behavior with GDB/MI's =library-loaded
events: The host-name="<path>" part of the event is now showing the
target-side path instead of the host-side path.

This revert affects Darwin and AIX systems, however, where the BFD
is either artificial or icomplete, leading to the outputt of
"info shared" not containing the information we'd like. For instance,
on Darwin, we would see:

    (top-gdb) info shared
    From                To                  Syms Read   Shared Object Library
    0x00007fff8d060de4  0x00007fff8d09ce1f  Yes (*)     i386:x86-64
    0x00007fff8af08b10  0x00007fff8b1c6f73  Yes (*)     i386:x86-64

To compensate for that, we overwrite the filename of the associated bfd.

gdb/ChangeLog:

	Revert the following commit:
	* solib.c (solib_map_sections): Remove code overwriting
	SO->SO_NAME with the bfd's filename.

	Make the following changes required after the revert above:
	* solib-aix.c (solib_aix_bfd_open): Set the filename of the
	returned bfd to a copy of the synthetic pathname.
	* solib-darwin.c (darwin_bfd_open): Set the filename of the
	returned bfd to a copy of PATHNAME.