x86/cet: fix shadow stack test scripts

Some shadow stack test scripts use the '==' operator with the 'test'
command to validate exit codes resulting in the following error:

  sysdeps/x86_64/tst-shstk-legacy-1e.sh: 31: test: 139: unexpected operator

The '==' operator is invalid for the 'test' command, use '-eq' like the
previous call to 'test'.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
(cherry picked from commit 155bb9d0)