This is a simple, fair spinlock.  Specifically it doesn't have all the
subtle memory model dependencies that qspinlock has, which makes it more
suitable for simple systems as it is more likely to be correct.  It is
implemented entirely in terms of standard atomics and thus works fine
without any arch-specific code.

This replaces the existing asm-generic/spinlock.h, which just errored
out on SMP systems.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>