RISC-V: Prohibit the 'Zcf' extension on RV64

As per:
<riscvarchive/riscv-code-size-reduction#221>,
the 'Zcf' extension does not exist on RV64.  This is reflected on the
version 1.0.4-1 of the code size reduction specification:
<https://github.com/riscv/riscv-code-size-reduction/releases/tag/v1.0.4-1>.

This commit prohibits the combination: RV64 (or any ISA with XLEN > 32)
and the 'Zcf' extension.

bfd/ChangeLog:

	* elfxx-riscv.c (riscv_parse_check_conflicts): Prohibit
	combination of RV64 and 'Zcf'.

gas/ChangeLog:

	* testsuite/gas/riscv/march-fail-rv64i_zcf.d: New test.
	* testsuite/gas/riscv/march-fail-rv64i_zcf.l: Likewise.

bfd/elfxx-riscv.c

@@ -1946,6 +1946,13 @@ riscv_parse_check_conflicts (riscv_parse_subset_t *rps)
       rps->error_handler (_("rv%d does not support the `q' extension"), xlen);
       no_conflict = false;
     }
+  if (riscv_lookup_subset (rps->subset_list, "zcf", &subset)
+      && xlen > 32)
+    {
+      rps->error_handler
+	(_("rv%d does not support the `zcf' extension"), xlen);
+      no_conflict = false;
+    }
   if (riscv_lookup_subset (rps->subset_list, "zfinx", &subset)
       && riscv_lookup_subset (rps->subset_list, "f", &subset))
     {

gas/testsuite/gas/riscv/march-fail-rv64i_zcf.d

@@ -0,0 +1,3 @@
+#as: -march=rv64i_zcf
+#source: empty.s
+#error_output: march-fail-rv64i_zcf.l

gas/testsuite/gas/riscv/march-fail-rv64i_zcf.l

@@ -0,0 +1,2 @@
+.*Assembler messages:
+.*Error: .*rv64 does not support the `zcf' extension