amd64: Fix two typos of loader tunables

To match the sysctl MIBs and document entries in security(7).

Fixes: 2dec2b4a34b4 amd64: flush L1 data cache on syscall return with an error
Fixes: 17edf152e556 Control for Special Register Buffer Data Sampling mitigation

Reviewed by: kib
MFC after: 1 day
Differential Revision: https://reviews.freebsd.org/D42249