gdb: First cut at RISC-V support

* Replace inclusion of "gdb_string.h" with <string.h>.
* riscv_read_pc: Declare `pc' as signed to conform to the type signature
  of regcache_cooked_read_signed().
* riscv_return_value: Update type of second parameter.
* riscv_pseudo_register_read: Update with non-void return type.
* gdb_print_insn_riscv: Replace call to print_insn_little_riscv() with
  print_insn_riscv().
gdb: Fix segfault issue and update register order/numbers to complete the standard CSR set
gdb: Fix debug command and update info registers display

* Rename frame_register_read to deprecated_frame_register_read.
gdb: Update breakpoint support
gdb: Move back to a default style of info register display

* Rename frame_register_read to deprecated_frame_register_read.
gdb: Remove unused functions and use alias names for register name display
gdb: Fix size representation of count and compare registers
gdb: Fix size representation of other supervisor registers
gdb: Update the register display to show fields of FCSR register
gdb: Make the FCSR register 32 bits wide