sys: Build arm64 per-thread SSP with GCC

It has been supported since GCC 9. It is unlikely anything older than
that will build the kernel so mark it as supported by GCC.

Reviewed by: brooks, jhb
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45267

(cherry picked from commit c2628accc029a07eb8ff73b6df7f7d6af194ca2d)