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.