ggml-cpu: cmake: append xsmtvdotii march for SpacemiT IME (#22317)

* ggml-cpu: cmake: append xsmtvdotii march for SpacemiT IME

When GGML_CPU_RISCV64_SPACEMIT=ON is set, ime1_kernels.cpp contains
inline asm for the vmadot family which requires the xsmtvdotii custom
extension.(problem can see in some blogs and make sure in K3 platform)
The current CMakeLists does not include xsmtvdotii, so any toolchain
that honours the explicit -march (tested with SpacemiT GCC 15.2) fails
at the assembler stage:

  Error: unrecognized opcode `vmadot v16,v14,v0',
         extension `xsmtvdotii' required

Append _xsmtvdotii to MARCH_STR when GGML_CPU_RISCV64_SPACEMIT is
enabled so the IME path can actually build with a capable toolchain.
No effect on builds that leave GGML_CPU_RISCV64_SPACEMIT off.

toolchain from https://www.spacemit.com/community/resources-download/Tools

* Update ggml/src/ggml-cpu/CMakeLists.txt

Co-authored-by: alex-spacemit <jinghui.huang@spacemit.com>

---------

Co-authored-by: alex-spacemit <jinghui.huang@spacemit.com>

ggml/src/ggml-cpu/CMakeLists.txt

@@ -485,6 +485,13 @@ function(ggml_add_cpu_backend_variant_impl tag_name)
             if (GGML_RV_ZIHINTPAUSE)
                 string(APPEND MARCH_STR "_zihintpause")
             endif()
+            if (GGML_CPU_RISCV64_SPACEMIT)
+                # `xsmtvdotii' is only required for GCC >= 15.
+                if (CMAKE_C_COMPILER_ID STREQUAL "GNU" AND
+                    CMAKE_C_COMPILER_VERSION VERSION_GREATER_EQUAL 15)
+                    string(APPEND MARCH_STR "_xsmtvdotii")
+                endif()
+            endif()
 
             list(APPEND ARCH_FLAGS "-march=${MARCH_STR}" -mabi=lp64d)
         else()