xlat: add PR_SPEC_INDIRECT_BRANCH to pr_spec_cmds

xlat: add PR_SPEC_INDIRECT_BRANCH to pr_spec_cmds

* xlat/pr_spec_cmds.in (PR_SPEC_INDIRECT_BRANCH): New constant,
introduced by Linux commit v4.20-rc5~4^2~3.
* prctl.c (SYS_FUNC(prctl)) <case PR_GET_SPECULATION_CTRL, case
PR_SET_SPECULATION_CTRL>: Add PR_SPEC_INDIRECT_BRANCH handling.
* tests/prctl-spec-inject.c (main): Add PR_SPEC_INDIRECT_BRANCH decoding
checks, update expected output.
* NEWS: Mention this.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>