amd64: assume the kernel supports RDFSBASE and RDGSBASE

[brooks]

Summary

There is no need to support pre-12.0 (and thus pre-ino64) kernels in a
15.x libc.

Continue to check if the CPU supports these features before using them
and fall back as required.

[kib]

sys/sys/param.h

100 ↗

(On Diff #156655)

Please keep the define.