[SYCL][NATIVECPU][DOCS] add Native CPU info to GettingStartedGuide (#19768)

Adding Native CPU to the startup guide and linking the design doc,
adding mention of -march option in design doc

Author: uwedolinsky

sycl/doc/GetStartedGuide.md

@@ -12,6 +12,7 @@ and a wide range of compute accelerators such as GPU and FPGA.
     * [Build DPC++ toolchain with support for NVIDIA CUDA](#build-dpc-toolchain-with-support-for-nvidia-cuda)
     * [Build DPC++ toolchain with support for HIP AMD](#build-dpc-toolchain-with-support-for-hip-amd)
     * [Build DPC++ toolchain with support for HIP NVIDIA](#build-dpc-toolchain-with-support-for-hip-nvidia)
+    * [Build DPC++ toolchain with support for Native CPU](#build-dpc-toolchain-with-support-for-native-cpu)
     * [Build DPC++ toolchain with support for ARM processors](#build-dpc-toolchain-with-support-for-arm-processors)
     * [Build DPC++ toolchain with additional features enabled that require runtime/JIT compilation](#build-dpc-toolchain-with-additional-features-enabled-that-require-runtimejit-compilation)
     * [Build DPC++ toolchain with device image compression support](#build-dpc-toolchain-with-device-image-compression-support)
@@ -123,6 +124,7 @@ flags can be found by launching the script with `--help`):
 * `--hip-platform` -> select the platform used by the hip backend, `AMD` or
   `NVIDIA` (see [HIP AMD](#build-dpc-toolchain-with-support-for-hip-amd) or see
   [HIP NVIDIA](#build-dpc-toolchain-with-support-for-hip-nvidia))
+* `--native_cpu` -> use the Native CPU backend (see [Native CPU](#build-dpc-toolchain-with-support-for-native-cpu))
 * `--enable-all-llvm-targets` -> build compiler (but not a runtime) with all
   supported targets
 * `--shared-libs` -> Build shared libraries
@@ -297,6 +299,13 @@ as well as the CUDA Runtime API to be installed, see [NVIDIA CUDA Installation
 Guide for
 Linux](https://docs.nvidia.com/cuda/cuda-installation-guide-linux/index.html).
 
+### Build DPC++ toolchain with support for Native CPU
+
+Native CPU is a cpu device which by default has no other dependency than DPC++. This device works with all cpu targets supported by the DPC++ runtime.
+Supported targets include x86, Aarch64 and riscv_64.
+
+To enable Native CPU in a DPC++ build just add `--native_cpu` to the set of flags passed to `configure.py`.
+
 ### Build DPC++ toolchain with support for ARM processors
 
 There is no continuous integration for this, and there are no guarantees for supported platforms or configurations.
@@ -696,6 +705,13 @@ clang++ -fsycl -fsycl-targets=nvptx64-nvidia-cuda \
   simple-sycl-app.cpp -o simple-sycl-app-cuda.exe
 ```
 
+When building for Native CPU use the SYCL target native_cpu:
+
+```bash
+clang++ -fsycl -fsycl-targets=native_cpu simple-sycl-app.cpp -o simple-sycl-app.exe
+```
+More Native CPU build options can be found in [SYCLNativeCPU.md](design/SYCLNativeCPU.md).
+
 **Linux & Windows (64-bit)**:
 
 ```bash

sycl/doc/design/SYCLNativeCPU.md

@@ -105,6 +105,8 @@ Whole Function Vectorization is enabled by default, and can be controlled throug
 * `-mllvm -sycl-native-cpu-no-vecz`: disable Whole Function Vectorization.
 * `-mllvm -sycl-native-cpu-vecz-width`: sets the vector width to the specified value, defaults to 8.
 
+The `-march=` option can be used to select specific target cpus which may improve performance of the vectorized code.
+
 For more details on how the Whole Function Vectorizer is integrated for SYCL Native CPU, refer to the [Technical details](#technical-details) section.
 
 # Code coverage