selftests: x86: add missing executables to .gitignore

Executables that are common for both x86_32 and x86_64 are missing
from .gitignore. Add them.

Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/x86/.gitignore

 *_32
 *_64
+single_step_syscall
+sysret_ss_attrs
+syscall_nt
+ptrace_syscall
+test_mremap_vdso
+check_initial_reg_state
+sigreturn
+ldt_gdt
+iopl
+mpx-mini-test
+ioperm
+protection_keys
+test_vdso