PR gdb/27570: missing support for debuginfod in core_target::build_file_mappings

Add debuginfod support to core_target::build_file_mappings and
locate_exec_from_corefile_build_id to enable the downloading of
missing executables and shared libraries referenced in core files.

Also add debuginfod support to solib_map_sections so that previously
downloaded shared libraries can be retrieved from the local debuginfod
cache.

When core file shared libraries are found locally, verify that their
build-ids match the corresponding build-ids found in the core file.
If there is a mismatch, attempt to query debuginfod for the correct
build and print a warning if unsuccessful:

  warning: Build-id of /lib64/libc.so.6 does not match core file.

Also disable debuginfod when gcore invokes gdb.  Debuginfo is not
needed for core file generation so debuginfod queries will slow down
gcore unnecessarily.