tools/nolibc/stdio: let perror work when NOLIBC_IGNORE_ERRNO is set

tools/nolibc/stdio: let perror work when NOLIBC_IGNORE_ERRNO is set

[ Upstream commit c485ca3aff2442adea4c08ceb5183e671ebed22a ]

There is no errno variable when NOLIBC_IGNORE_ERRNO is defined. As such,
simply print the message with "unknown error" rather than the integer
value of errno.

Fixes: acab7bcdb1bc ("tools/nolibc/stdio: add perror() to report the errno value")
Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>