gdb/riscv: correct dwarf to gdb register number mapping

gdb/riscv: correct dwarf to gdb register number mapping

According to the riscv psabi, the mapping relationship between the
DWARF registers and the machine registers is as follows:

  DWARF Number | Register Name | Description
  0 - 31       | x0 - x31      | Integer Registers
  32 - 63      | f0 - f31      | Floating-point Registers

This is not modelled quite right in riscv_dwarf_reg_to_regnum, the
DWARF register numbers 31 and 63 are not handled correctly due to a
use of '<' instead of '<='.  This commit fixes this issue.