libc: gate aarch64 atomic.h kernel-only includes so ZFS userspace builds

libc/arch/aarch64/atomic.h pulls in OSv kernel-only headers (the
FreeBSD-derived machine/atomic.h and the bsd/cddl opensolaris sys/types.h),
which are only on the include path for kernel/bsd objects. Userspace
translation units that resolve <atomic.h> to this file (the OpenZFS
libspl/libzfs sources on aarch64) do not have them and do not need them.
Gate those includes on the kernel build and use the compiler atomic builtin
in the userspace path. The x86_64 variant has no such includes, so this is
aarch64-only and leaves x86_64 byte-identical.

Author: gburd

libc/arch/aarch64/atomic.h

@@ -9,8 +9,19 @@
 #define _INTERNAL_ATOMIC_H
 
 #include <stdint.h>
+/*
+ * These two headers pull in OSv kernel-only machinery (the FreeBSD-derived
+ * machine/atomic.h and the old bsd/cddl opensolaris sys/types.h). They are only
+ * on the include path for kernel/bsd objects. Userspace translation units that
+ * resolve <atomic.h> to this file (e.g. the OpenZFS libspl/libzfs sources on
+ * aarch64) do not have them available and do not need them, so gate them on the
+ * kernel build. The x64 variant of this file has no such includes; this change
+ * is aarch64-only and leaves the x86_64 build byte-identical.
+ */
+#if defined(_KERNEL) || defined(__OSV_CORE__)
 #include <bsd/sys/cddl/compat/opensolaris/sys/types.h>
 #include <machine/atomic.h>
+#endif
 
 static inline int a_ctz_64(register uint64_t x)
 {
@@ -26,7 +37,12 @@ static inline int a_ctz_l(unsigned long x)
 
 static inline int a_fetch_add(volatile int *x, int v)
 {
+#if defined(_KERNEL) || defined(__OSV_CORE__)
     return atomic_fetchadd_int((unsigned int *)x, (unsigned int)v);
+#else
+    /* Userspace: machine/atomic.h is unavailable; use the LSE/LL-SC builtin. */
+    return __atomic_fetch_add(x, v, __ATOMIC_SEQ_CST);
+#endif
 }
 
 static inline void a_crash()