gdb/riscv: Loop over all registers for 'info all-registers'

Currently the 'info all-registers' command only loops over those
registers that are known to GDB.  Any registers that are unknown, that
is, are mentioned in the target description, but are not something GDB
otherwise knows, will not be displayed.

This feels wrong, so this commit fixes this mistake.  The output of
'info all-registers' now matches 'info registers all'.

gdb/ChangeLog:

	* riscv-tdep.c (riscv_print_registers_info): Loop over all
	registers, not just the known core set of registers.

gdb/testsuite/ChangeLog:

	* gdb.arch/riscv-tdesc-regs.exp: New test cases.