Add 'xt' prefix for Xuantie vendor extension

This patch is adding 'xt' to the list of vendor prefixes. The relationship between 'xt' and 'th' has been added in the NOTE

Signed-off-by: joshua-arch1 <cooper.joshua@linux.alibaba.com>

Author: joshua-arch1

src/toolchain-conventions.adoc

@@ -407,6 +407,7 @@ before modifying this table.
 |T-Head                 | th              | https://www.t-head.cn/
 |Tenstorrent            | tt              | https://www.tenstorrent.com/
 |Ventana Micro Systems  | vt              | https://www.ventanamicro.com/
+|Xuantie                | xt              | https://www.xrvm.cn/
 |===
 
 NOTE: Vendor prefixes are case-insensitive.
@@ -415,6 +416,8 @@ NOTE: The Nuclei instruction prefix `xl` is an abbreviation of "XinLai", which i
 
 NOTE: OpenHW cores are all branded as CORE-V, hence the prefix.
 
+NOTE: Xuantie, which defined the XThead vendor extensions, was one of T-Head products but now separated out.
+
 === List of vendor identifiers
 Vendor identifiers are dummy symbols used in the corresponding `R_RISCV_VENDOR`
 relocation (irrespective of ELF class/XLEN) and must be unique amongst all