gdb/riscv: Record information about unknown tdesc registers

Making use of the previous commit, record information about unknown
registers in the target description, and use this to resolve two
issues.

1. Some targets (QEMU) are reporting three register fflags, frm, and
   fcsr, twice, once in the FPU feature, and once in the CSR feature.
   GDB does create two registers with identical names, but this
   is (sort of) fine, we only ever use the first one, and as both
   registers access the same target state things basically work OK.
   The only real problem is that the register names show up twice in
   'info registers all' output.

   In this commit we spot the duplicates of these registers and then
   return NULL when asked for the name of these registers.  This
   causes GDB to hide these registers from the user, fixing this
   problem.

2. Some targets (QEMU) advertise CSRs that GDB then can't read.  The
   problem is these targets also say these CSRs are part of the
   save/restore register groups.

   This means that before an inferior call GDB tries to save all of
   these CSRs, and a failure to read one causes the inferior call to
   be abandoned.

   We already work around this issue to some degree, known CSRs are
   removed from the save/restore groups, despite what the target might
   say.  However, any unknown CSRs are (currently) not removed in this
   way.

   After this commit we keep a log of the register numbers for all
   unknown CSRs, then when asked about the register groups, we
   override the group information for unknown CSRs, removing them from
   the save and restore groups.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_register_name): Return NULL for duplicate
	fflags, frm, and fcsr registers.
	(riscv_register_reggroup_p): Remove unknown CSRs from save and
	restore groups.
	(riscv_tdesc_unknown_reg): New function.
	(riscv_gdbarch_init): Pass riscv_tdesc_unknown_reg to
	tdesc_use_registers.
	* riscv-tdep.h (struct gdbarch_tdep): Add
	unknown_csrs_first_regnum, unknown_csrs_count,
	duplicate_fflags_regnum, duplicate_frm_regnum, and
	duplicate_fcsr_regnum fields.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-tdesc-regs.exp: Extend test case.