Merge commit 'ce929fe84f9c453263af379f3b255ff8eca01d48'

Import CK as of commit 2265c7846f4ce667f5216456afe2779b23c3e5f7.

Remove FreeBSD/armv4 specific bits from CK.

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.

MFV CK@r336629: Import CK as of commit 1c1f9901c2dea7a883342cd03d3906a1bc482583

This adds CK_SLIST_INSERT_PREVPTR and CK_SLIST_REMOVE_PREVPTR macros
as well as ck_pr_dec_is_zero family of functions.

MFC after: 3 weeks

Import CK as of commit b19ed4c6a56ec93215ab567ba18ba61bf1cfbac8
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

Merge CK as of commit 255a47553aa5e8d0bb5f8eec63acac7f4c25a6d8, mostly
to make sure we don't use any FP instruction.

Add FreeBSD-specific files.

Import Concurrency Kit in the kernel.
CK is a toolkit providing different lockfree algorithms/data structures.
More information can be found here : www.concurrencykit.org

Remove FreeBSD/armv4 specific bits from CK.

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.

Add FreeBSD-specific files.