gdb/riscv: place unknown csrs into the correct register groups

Unknown riscv CSRs should not be in the 'general' group, but should be
in the system and csr register groups.

To see this in action connect to QEMU, this target advertises two
registers dscratch and mucounteren which are unknown to GDB (these are
legacy CSRs).  Before this commit these registers would show up in the
output of:

  (gdb) info registers
  ....
  dscratch       Could not fetch register "dscratch"; remote failure reply 'E14'
  mucounteren    Could not fetch register "mucounteren"; remote failure reply 'E14'

Ignore the errors, this is just a QEMU annoyance, it advertises these
CSRs, but doesn't actually let GDB read them.  These registers don't
show up in the output of either:

  (gdb) info registers csr
  (gdb) info registers system

After this commit this situation is reveresed, which makes more sense
to me.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_is_unknown_csr): New function,
	implementation moved from riscv_register_reggroup_p.
	(riscv_register_reggroup_p): Update group handling for unknown
	CSRs.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-tdesc-regs.exp (get_expected_result): New proc,
	update test to use this.

gdb/riscv-tdep.c

@@ -969,6 +969,18 @@ riscv_is_regnum_a_named_csr (int regnum)
     }
 }
 
+/* Return true if REGNUM is an unknown CSR identified in
+   riscv_tdesc_unknown_reg for GDBARCH.  */
+
+static bool
+riscv_is_unknown_csr (struct gdbarch *gdbarch, int regnum)
+{
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+  return (regnum >= tdep->unknown_csrs_first_regnum
+	  && regnum < (tdep->unknown_csrs_first_regnum
+		       + tdep->unknown_csrs_count));
+}
+
 /* Implement the register_reggroup_p gdbarch method.  Is REGNUM a member
    of REGGROUP?  */
 
@@ -986,13 +998,21 @@ riscv_register_reggroup_p (struct gdbarch  *gdbarch, int regnum,
     {
       /* Any extra registers from the CSR tdesc_feature (identified in
 	 riscv_tdesc_unknown_reg) are removed from the save/restore groups
-	 as some targets (QEMU) report CSRs which then can't be read.  */
-      struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
-      if ((reggroup == restore_reggroup || reggroup == save_reggroup)
-	  && regnum >= tdep->unknown_csrs_first_regnum
-	  && regnum < (tdep->unknown_csrs_first_regnum
-		       + tdep->unknown_csrs_count))
-	return 0;
+	 as some targets (QEMU) report CSRs which then can't be read and
+	 having unreadable registers in the save/restore group breaks
+	 things like inferior calls.
+
+	 The unknown CSRs are also removed from the general group, and
+	 added into both the csr and system group.  This is inline with the
+	 known CSRs (see below).  */
+      if (riscv_is_unknown_csr (gdbarch, regnum))
+	{
+	  if (reggroup == restore_reggroup || reggroup == save_reggroup
+	       || reggroup == general_reggroup)
+	    return 0;
+	  else if (reggroup == system_reggroup || reggroup == csr_reggroup)
+	    return 1;
+	}
 
       /* This is some other unknown register from the target description.
 	 In this case we trust whatever the target description says about