FreeBSD: make adjustments for the standalone environment

In FreeBSD, there are three compile environments that are supported:
user land, the kernel and the bootloader / standalone. Adjust the
headers to compile in the standalone environment. Limit kernel-only
items from view when _STANDALONE is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Warner Losh <imp@FreeBSD.org>
Closes #10998

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

@@ -29,6 +29,8 @@
 #ifndef _OPENSOLARIS_SYS_ATOMIC_H_
 #define	_OPENSOLARIS_SYS_ATOMIC_H_
 
+#ifndef _STANDALONE
+
 #include <sys/types.h>
 #include <machine/atomic.h>
 
@@ -179,4 +181,13 @@ atomic_cas_ptr(volatile void *target, void *cmp,  void *newval)
 }
 #endif	/* !defined(COMPAT_32BIT) && defined(__LP64__) */
 
+#else /* _STANDALONE */
+/*
+ * sometimes atomic_add_64 is defined, sometimes not, but the
+ * following is always right for the boot loader.
+ */
+#undef atomic_add_64
+#define	atomic_add_64(ptr, val) *(ptr) += val
+#endif /* !_STANDALONE */
+
 #endif	/* !_OPENSOLARIS_SYS_ATOMIC_H_ */