stty: print the speed on big-endian PowerPC BSDs

[pkubaj]

The baud rate is printed by one of three cfg-gated branches. The first
one is for "BSDs and Linux (not ppc/big-endian ppc64)", but its PowerPC
exclusion was a second #[cfg] stacked on top of
any(target_os = "linux", bsd), so it applied to the BSDs as well. The
second branch is Linux-only and the third covers everything that is
neither Linux nor a BSD. On FreeBSD/powerpc64 (big-endian) and
FreeBSD/powerpc none of the three was compiled in, and stty silently
left "speed N baud;" out of its output.

The PowerPC special case is only needed on Linux, where those targets
get a BaudRate enum instead of a number. On the BSDs the baud rate is a
plain u32 on every architecture, which string_to_baud() already relies
on with a bare #[cfg(bsd)]. Restrict the architecture exclusion to
Linux so that the BSDs always take the u32 path.

Fixes test_stty::test_basic and test_stty::test_stty_uses_stdin on
FreeBSD/powerpc64.

[sylvestre]

Please split that into two pr

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/date/resolution (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/retry (passes in this run but fails in the 'main' branch)