[gdb/tdep, aarch64] Remove fp and sp reg aliases, add x31 reg alias

[gdb/tdep, aarch64] Remove fp and sp reg aliases, add x31 reg alias

In aarch64-tdep.c we find these register aliases:
...
{
  /* 64-bit register names.  */
  {"fp", AARCH64_FP_REGNUM},
  {"lr", AARCH64_LR_REGNUM},
  {"sp", AARCH64_SP_REGNUM},
...

The sp alias is superfluous, because the canonical name of x31 is already sp.

The fp alias is superfluous, because it's already taken by the default meaning
of fp, assigned here in _initialize_frame_reg:
...
  user_reg_add_builtin ("fp", value_of_builtin_frame_fp_reg, NULL);
...

Fix this by removing the fp and sp aliases.

While we're at it, add an x31 alias for sp.

Approved-By: Luis Machado <luis.machado@arm.com>
Tested on aarch64-linux.
PR tdep/30012
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30012