RISC-V/Linux/native: Factor out target description determination

In preparation for RISC-V/Linux `gdbserver' support factor out parts of
native target description determination code that can be shared between
the programs.

	gdb/
	* nat/riscv-linux-tdesc.h: New file.
	* nat/riscv-linux-tdesc.c: New file, taking code from...
	* riscv-linux-nat.c (riscv_linux_nat_target::read_description):
	... here.
	* configure.nat <linux> <riscv*>: Add nat/riscv-linux-tdesc.o to
	NATDEPFILES.

gdb/nat/riscv-linux-tdesc.c

@@ -0,0 +1,83 @@
+/* GNU/Linux/RISC-V native target description support for GDB.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#include "gdbsupport/common-defs.h"
+
+#include "gdb_proc_service.h"
+#include "arch/riscv.h"
+#include "elf/common.h"
+#include "nat/gdb_ptrace.h"
+#include "nat/riscv-linux-tdesc.h"
+
+#include <sys/uio.h>
+
+/* Work around glibc header breakage causing ELF_NFPREG not to be usable.  */
+#ifndef NFPREG
+# define NFPREG 33
+#endif
+
+/* Determine XLEN and FLEN and return a corresponding target description.  */
+
+const struct target_desc *
+riscv_linux_read_description (int tid)
+{
+  struct riscv_gdbarch_features features;
+  elf_fpregset_t regs;
+  int flen;
+
+  /* Figuring out xlen is easy.  */
+  features.xlen = sizeof (elf_greg_t);
+
+  /* Start with no f-registers.  */
+  features.flen = 0;
+
+  /* How much worth of f-registers can we fetch if any?  */
+  for (flen = sizeof (regs.__f.__f[0]); ; flen *= 2)
+    {
+      size_t regset_size;
+      struct iovec iov;
+
+      /* Regsets have a uniform slot size, so we count FSCR like
+	 an FP data register.  */
+      regset_size = ELF_NFPREG * flen;
+      if (regset_size > sizeof (regs))
+	break;
+
+      iov.iov_base = &regs;
+      iov.iov_len = regset_size;
+      if (ptrace (PTRACE_GETREGSET, tid, NT_FPREGSET,
+		  (PTRACE_TYPE_ARG3) &iov) == -1)
+	{
+	  switch (errno)
+	    {
+	    case EINVAL:
+	      continue;
+	    case EIO:
+	      break;
+	    default:
+	      perror_with_name (_("Couldn't get registers"));
+	      break;
+	    }
+	}
+      else
+	features.flen = flen;
+      break;
+    }
+
+  return riscv_create_target_description (features);
+}