RISC-V: Clarify the naming rules of vendor operands.

RISC-V: Clarify the naming rules of vendor operands.

The vendor operands should be named starting with `X', and preferably the
second letter (or multiple following letters) is enough to differentiate
them from other vendors.

Therefore, added letter `t' after `X' for t-head operands, to differentiate
from future different vendor's operands.

bfd/
	* elfxx-riscv.c (riscv_supported_vendor_x_ext): Removed the vendor
	document link since it should already be recorded in the
	gas/doc/c-riscv.texi.
gas/
	* config/tc-riscv.c (validate_riscv_insn): Added `t' after `X' for
	t-head operands.  Minor updates for indents and comments.
	(riscv_ip): Likewise.
	* doc/c-riscv.texi: Minor updates.
opcodes/
	* riscv-dis.c (print_insn_args): Added `t' after `X' for t-head
	operands.  Minor updates for indents and comments.
	* riscv-opc.c (riscv_opcode): Likewise.