[gdb/testsuite] Fix gdb.base/ending-run.exp on manjaro linux

On aarch64-linux, using the manjaro linux distro, I run into:
...
(gdb) next^M
32      }^M
(gdb) next^M
0x0000fffff7d67b80 in ?? () from /usr/lib/libc.so.6^M
(gdb) FAIL: gdb.base/ending-run.exp: step out of main
...

What happens here is described in detail in this clause:
...
    -re "0x.*\\?\\? \\(\\) from /lib/powerpc.*$gdb_prompt $" {
	# This case occurs on Powerpc when gdb steps out of main and the
	# needed debug info files are not loaded on the system, preventing
	# GDB to determine which function it reached (__libc_start_call_main).
	# Ideally, the target system would have the necessary debugging
	# information, but in its absence, GDB's behavior is as expected.
	...
    }
...
but the clause only matches for powerpc.

Fix this by:
- making the regexp generic enough to also match /usr/lib/libc.so.6, and
- updating the comment to not mention powerpc.

Tested on aarch64-linux.

PR testsuite/31450
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31450

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