devel/llvm18: Backport __stack_chk_guard fix

Backport upstream commit commit 26464f2662d1 which marks __stack_chk_guard
dso_local except for PPC64 to fix loading an x86 kernel build with
--fstack-protector.

Also update to 18.1.8. From a packaging perspective this only bumps the
version number since it only touchs libc++ which we don't ship. As such,
I'd waited to bundle this bump with a functional change.

Drop releases.llvm.org, the LLVM project seems to nonlonger update it.

Co-authored by: Ahmad Khalifa <ahmadkhalifa570@gmail.com>
Pull Request: https://github.com/freebsd/freebsd-ports/pull/283

(cherry picked from commit 772e9ff110df8a8485f4e417c648b626dd829e9e)