[master] fix build error on x86_64 solaris

CHANGES

@@ -1,3 +1,5 @@
+4437.	[port]		Corrected a build error on x86_64 Solaris. [RT #42150]
+
 4436.	[bug]		Fixed a regression introduced in change #4337 which
 			caused signed domains with revoked KSKs to fail
 			validation. [RT #42147]

lib/isc/x86_64/include/isc/atomic.h

@@ -87,12 +87,26 @@ isc_atomic_store(isc_int32_t *p, isc_int32_t val) {
 		"lock;"
 #endif
 		"xchgl (%rax), %edx\n"
-		/*
-		 * XXX: assume %rax will be used as the return value.
-		 */
 		);
 }
 
+#ifdef ISC_PLATFORM_HAVEATOMICSTOREQ
+static void
+isc_atomic_storeq(isc_int64_t *p, isc_int64_t val) {
+	UNUSED(p);
+	UNUSED(val);
+
+	__asm (
+		"movq %rdi, %rax\n"
+		"movq %rsi, %rdx\n"
+#ifdef ISC_PLATFORM_USETHREADS
+		"lock;"
+#endif
+		"xchgq (%rax), %rdx\n"
+		);
+}
+#endif
+
 static isc_int32_t
 isc_atomic_cmpxchg(isc_int32_t *p, isc_int32_t cmpval, isc_int32_t val) {
 	UNUSED(p);