freebsd: Use compiler.h from FreeBSD's base's linuxkpi

The FreeBSD linux/compiler.h in OpenZFS was copied from a very old
version of FreeBSD's linuxkpi's linux/compiler.h. There's no need for
this duplication. Use FreeBSD's linuxkpi version instead, and provide
zfs_fallthrough to augment it (it's all that's needed). Use #pragma once
to avoid naming issues for guard variables. Since this is a complete
rewrite, use my copyright here (the original code in FreeBSD still
credits everybody). This works back at least to FreeBSD 12.4, which
is not out of support, and all newer releases.

Remove extra copies of macros that were defined elsewhere, but are now
properly defined in LinuxKPI so are redundant.

Sponsored-by: Netflix
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Closes #16650

include/os/freebsd/spl/sys/ccompat.h

@@ -70,15 +70,6 @@ hlist_del(struct hlist_node *n)
 		n->next->pprev = n->pprev;
 }
 	/* BEGIN CSTYLED */
-#define	READ_ONCE(x) ({			\
-	__typeof(x) __var = ({		\
-		barrier();		\
-		ACCESS_ONCE(x);		\
-	});				\
-	barrier();			\
-	__var;				\
-})
-
 #define	HLIST_HEAD_INIT { }
 #define	HLIST_HEAD(name) struct hlist_head name = HLIST_HEAD_INIT
 #define	INIT_HLIST_HEAD(head) (head)->first = NULL

include/os/freebsd/spl/sys/debug.h

@@ -95,10 +95,6 @@ spl_assert(const char *buf, const char *file, const char *func, int line)
 #ifndef expect
 #define	expect(expr, value) (__builtin_expect((expr), (value)))
 #endif
-#ifndef __linux__
-#define	likely(expr)   expect((expr) != 0, 1)
-#define	unlikely(expr) expect((expr) != 0, 0)
-#endif
 
 #define	PANIC(fmt, a...)						\
 	spl_panic(__FILE__, __FUNCTION__, __LINE__, fmt, ## a)