[gdb/testsuite] Factor out dump_info in gdb.testsuite/dump-system-info.exp

Factor out new proc dump_info in test-case gdb.testsuite/dump-system-info.exp,
and in the process:
- fix a few typos
- remove unnecessary "test -r /proc/cpuinfo"

Tested on x86_64-linux.

Co-Authored-By: Pedro Alves <pedro@palves.net>