prctl: add PR_GET_SPECULATION_CTRL/PR_SET_SPECULATION_CTRL decoding

prctl: add PR_GET_SPECULATION_CTRL/PR_SET_SPECULATION_CTRL decoding

* xlat/pr_spec_cmds.in: New fille.
* xlat/pr_spec_get_store_bypass_flags.in: Likewise.
* xlat/pr_spec_set_store_bypass_flags.in: Likewise.
* xlat/prctl_options.in (PR_GET_SPECULATION_CTRL,
PR_SET_SPECULATION_CTRL): New constants, introduced by Linux commit
v4.17-rc3-15-gb617cfc.
* prctl.c (SYS_FUNC(prctl)) <case PR_GET_SPECULATION_CTRL,
case PR_SET_SPECULATION_CTRL>: Implement decoding of new prctl options.
* tests/Makefile.am (check_PROGRAMS): Add prctl-spec-inject.
(DECODER_TESTS): Add prctl-spec-inject.test.
* tests/prctl-spec-inject.c: New file.
* tests/prctl-spec-inject.test: New test.
* tests/.gitignore: Add prctl-spec-inject.