xen/README: add compiler and binutils versions for RISCV-64

Considering that the Zbb extension is supported since GCC version 12 [1]
and that older GCC versions do not support Z extensions in -march (I haven't
faced this issue for GCC >=11.2), leading to compilation failures,
the baseline version for GCC is set to 12.2 and for GNU binutils to 2.39.

The GCC version is set to 12.2 instead of 12.1 because Xen's GitLab CI uses
Debian 12, which includes GCC 12.2 and GNU binutils 2.39.

[1] https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=149e217033f01410a9783c5cb2d020cf8334ae4c

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

README

@@ -48,6 +48,9 @@ provided by your OS distributor:
       - For ARM 64-bit:
         - GCC 5.1 or later
         - GNU Binutils 2.24 or later
+      - For RISC-V 64-bit:
+        - GCC 12.2 or later
+        - GNU Binutils 2.39 or later
     * POSIX compatible awk
     * Development install of zlib (e.g., zlib-dev)
     * Development install of Python 2.7 or later (e.g., python-dev)