Fix sigkill.stderr.exp for glibc-2.28.

Fix sigkill.stderr.exp for glibc-2.28.

glibc 2.28 filters out some bad signal numbers and returns
Invalid argument instead of passing such bad signal numbers
the kernel sigaction syscall. So we won't see such bad signal
numbers and won't print "bad signal number" ourselves.

Add a new memcheck/tests/sigkill.stderr.exp-glibc-2.28 to catch
this case.