gdb: use std::vector to store segments in symfile_segment_data

gdb: use std::vector to store segments in symfile_segment_data

Instead of maintaining two vectors, I added a small `segment` class
which holds both the base address and size of one segment and replaced
the two `segment_bases` and `segment_sizes` arrays with a single vector.

The rest of the changes are straightforward, no behavior changes are
expected.

gdb/ChangeLog:

	* symfile.h (struct symfile_segment_data) <struct segment>: New.
	<segments>: New.
	<segment_bases, segment_sizes>: Remove.
	* symfile.c (default_symfile_segments): Update.
	* elfread.c (elf_symfile_segments): Update.
	* remote.c (remote_target::get_offsets): Update.
	* solib-target.c (solib_target_relocate_section_addresses):
	Update.