A use of risbg instruction is not guarded by the corresponding
GENERAL_INSTR_EXT facility check.  This will result in an illegal
instruction when running on hardware without that facility installed
(i.e. z9).

R=jyan@ca.ibm.com,michael_dawson@ca.ibm.com,mbrandy@us.ibm.com
BUG=

Review URL: https://codereview.chromium.org/1848533006

Cr-Commit-Position: refs/heads/master@{#35206}