Now that armv4/v5 is gone, remove the bits that implemented atomic operations
by disabling interrupts.
Those were specific to FreeBSD and never reached upstream.
sparcv9 atomics compatible with the FreeBSD kernel by using instructions
which access the appropriate address space.
Atomic operations within the kernel must access the nucleus address space
instead of the default primary one. Without this change but the increased
use of CK in the kernel, machines started to panic after some minutes of
uptime due to an unresolvable fault in ck_pr_cas_64_value().
It should fix ck_pr_[load|store]_ptr on mips and riscv, make sure no
*fence instructions are used on i386, as older cpus don't support it, and
make sure we don't rely on gcc builtins that can lead to calls to
libatomic when linked with -O0.
MFC after: 1 week
