cmake: Add RISC-V compiler support

[lhpqaq]

#10400

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[lhpqaq]

(base)  build (master) ✗ cat ../toolchain.cmake
set(CMAKE_C_COMPILER /usr/bin/riscv64-linux-gnu-gcc)
set(CMAKE_CXX_COMPILER /usr/bin/riscv64-linux-gnu-g++)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR riscv64)

set(CMAKE_FIND_ROOT_PATH /path/to/your/sysroot)
set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)%
(base)  build (master) ✗ cmake -DCMAKE_TOOLCHAIN_FILE=../toolchain.cmake ..
-- The C compiler identification is GNU 13.2.0
-- The CXX compiler identification is GNU 13.2.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/riscv64-linux-gnu-gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/riscv64-linux-gnu-g++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Git: /usr/bin/git (found version "2.43.0")
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Success
-- Found Threads: TRUE
-- Warning: ccache not found - consider installing it for faster compilation or disable this warning with GGML_CCACHE=OFF
-- CMAKE_SYSTEM_PROCESSOR: riscv64
-- Found OpenMP_C: -fopenmp (found version "4.5")
-- Found OpenMP_CXX: -fopenmp (found version "4.5")
-- Found OpenMP: TRUE (found version "4.5")
-- OpenMP found
-- Using llamafile
-- RISC-V detected
-- Using runtime weight conversion of Q4_0 to Q4_0_x_x to enable optimized GEMM/GEMV kernels
-- Including CPU backend
CMake Warning at ggml/src/ggml-amx/CMakeLists.txt:106 (message):
  AMX requires x86 and gcc version > 11.0.  Turning off GGML_AMX.


-- Configuring done (1.0s)
-- Generating done (0.1s)
-- Build files have been written to: /home/lhp/llama.cpp/build

RISC-V detected