Enable CAS for byte and short on RISC-V

Issue #31 (libatomic_ops).

At least for gcc-7.2.0 does not define __GCC_HAVE_SYNC_COMPARE_AND_SWAP_1
and __GCC_HAVE_SYNC_COMPARE_AND_SWAP_2 macros but, nonetheless, all the
provided CAS operations are lock-free (except for double-word).

* src/Makefile.am (nobase_private_HEADERS): Add gcc/riscv.h entry.
* src/atomic_ops.h [__GNUC__ && !AO_USE_PTHREAD_DEFS
&& !__INTEL_COMPILER && __riscv]: Include gcc/riscv.h file.
* src/atomic_ops/sysdeps/gcc/riscv.h: New file (include generic.h).
* src/atomic_ops/sysdeps/gcc/riscv.h (AO_GCC_FORCE_HAVE_CAS): Define
before include generic.h (and undefined at the end of file); add
comment about double-word operations.

src/Makefile.am

@@ -91,6 +91,7 @@ nobase_private_HEADERS = atomic_ops/ao_version.h \
           atomic_ops/sysdeps/gcc/m68k.h \
           atomic_ops/sysdeps/gcc/mips.h \
           atomic_ops/sysdeps/gcc/powerpc.h \
+          atomic_ops/sysdeps/gcc/riscv.h \
           atomic_ops/sysdeps/gcc/s390.h \
           atomic_ops/sysdeps/gcc/sh.h \
           atomic_ops/sysdeps/gcc/sparc.h \

src/atomic_ops.h

@@ -329,9 +329,11 @@
 #   include "atomic_ops/sysdeps/gcc/avr32.h"
 # elif defined(__hexagon__)
 #   include "atomic_ops/sysdeps/gcc/hexagon.h"
+# elif defined(__riscv)
+#   include "atomic_ops/sysdeps/gcc/riscv.h"
 # elif defined(__tile__)
 #   include "atomic_ops/sysdeps/gcc/tile.h"
-# else /* __nios2__, __riscv, etc. */
+# else /* __nios2__, etc. */
 #   include "atomic_ops/sysdeps/gcc/generic.h"
 # endif
 #endif /* __GNUC__ && !AO_USE_PTHREAD_DEFS */

src/atomic_ops/sysdeps/gcc/riscv.h

@@ -0,0 +1,22 @@
+/*
+ * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
+ * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
+ *
+ * Permission is hereby granted to use or copy this program
+ * for any purpose,  provided the above notices are retained on all copies.
+ * Permission to modify the code and to distribute modified code is granted,
+ * provided the above notices are retained, and a notice that the code was
+ * modified is included with the above copyright notice.
+ */
+
+/* As of gcc-7.2.0, some __GCC_HAVE_SYNC_COMPARE_AND_SWAP_n are missing. */
+/* The operations are lock-free (even for the types smaller than word).  */
+#define AO_GCC_FORCE_HAVE_CAS
+
+/* While double-word atomic operations are provided by the compiler     */
+/* (which requires -latomic currently), they are not lock-free as       */
+/* riscv itself does not have the double-word atomic operations.        */
+
+#include "generic.h"
+
+#undef AO_GCC_FORCE_HAVE_CAS