gdb/riscv: write CSRs into baremetal core dumps

gdb/riscv: write CSRs into baremetal core dumps

Use the current target description to include CSRs into the RISC-V
baremetal core dumps.

Every CSR declared in the current target description will be included
in the core dump.

It will be critical for users that they have the same target
description in use when loading the core file as was in use when
writing the core file.  This should be fine if the user allows the
target description to be written into the core file.

In more detail, this commit adds a NT_RISCV_CSR note type.  The
contents of this section is a series of either 4-byte (on RV32
targets), or 8-byte (on RV64 targets) values.  Every CSR that is
mentioned in the current target description is written out in the
order the registers appear in the target description.  As a
consequence it is critical that the exact same target description,
including the same register order, is in use when the CSRs are loaded
from the core file.

gdb/ChangeLog:

	* riscv-none-tdep.c: Add 'user-regs.h' and 'target-description.h'
	includes.
	(riscv_csrset): New static global.
	(riscv_update_csrmap): New function.
	(riscv_iterate_over_regset_sections): Process CSRs.